Thank you for reporting the bug. Could you please try to reproduce it with the latest version of LibreOffice from https://www.libreoffice.org/download/libreoffice-fresh/ ? I have set the bug's status to 'NEEDINFO'. Please change it back to 'UNCONFIRMED' if the bug is still present in the latest version. Change to RESOLVED WORKSFORME, if the problem went away.

Could you please also reset your Libreoffice profile (https://wiki.documentfoundation.org/UserProfile) and re-test?

Didn't work with and without profile reset. (Also customised Ctrl+Shift+[ to check if it was something Ctrl+Shift related, but it worked normally)

Profile reset done as:
1. Start LibreOffice and select Help ▸ Restart in Safe Mode
2. Select Continue in Safe Mode (after restart is done and a modal dialog appears)

[New version info]
OS: Fedora 35 (5.16.19-200.fc35.x86_64)
GNOME Shell 41.4
Windowing system: Wayland
Version: 7.3.2.2 / LibreOffice Community
Build ID: 49f2b1bff42cfccbd8f788c8dc32c1c309559be0
CPU threads: 4; OS: Linux 5.16; UI render: default; VCL: gtk3
Locale: pt-BR (en_US.UTF-8); UI: en-US
Calc: threaded

Thank you for retesting.

I can't confirm with

Version: 7.3.3.1 (x64) / LibreOffice Community
Build ID: 1688991ca59a3ca1c74bc2176b274fba1b034928
CPU threads: 4; OS: Windows 10.0 Build 19044; UI render: Skia/Raster; VCL: win
Locale: de-DE (de_DE); UI: en-GB
Calc: CL

Tested with "Ctrl+Shift+," and with "Ctrl+Shift+.".

Tested again with profile reset (safe mode) on 

Version: 7.3.3.1 / LibreOffice Community
Build ID: 1688991ca59a3ca1c74bc2176b274fba1b034928
CPU threads: 4; OS: Linux 5.16; UI render: default; VCL: gtk3
Locale: en-US (en_US.UTF-8); UI: en-US
Calc: threaded

Still didn't work. Maybe it's an OS/GNOME specific error?

Version: 7.2.5.2 / LibreOffice Community
Build ID: 499f9727c189e6ef3471021d6132d4c694f357e5
CPU threads: 2; OS: Linux 5.15; UI render: default; VCL: gtk3
Locale: en-PH (en_PH.UTF-8); UI: en-US
Calc: threaded

Version: 7.3.4.2 / LibreOffice Community
Build ID: 30(Build:2)
CPU threads: 1; OS: Linux 5.15; UI render: default; VCL: gtk3
Locale: en-PH (en_PH.UTF-8); UI: en-US
Ubuntu package version: 1:7.3.2-0ubuntu2
Calc: threaded

Able to replicate the issue in the above versions. Also, I tried to check if other options would have the same behavior. I don’t have time to check all available options but please see the shortcuts I checked below.
All of them are set to show the LO About page for simplicity.

For clarity, I also followed the steps below. It's technically the same as the original reporter's, but a little detailed.

Steps to Reproduce:
1. Go to Tools > Customize > Keyboard
2. In the Shortcut Keys go to either Shift+Ctrl+, or Shift+Ctrl+.
3. Select any Category below by clicking an item (I picked Application)
4. Select any item from Function (I picked About LibreOffice)
5. Click Modify button on the upper right side of the Customize panel
6. Selected shortcut keys should be added in the Keys section in the lower right of the Customize panel
7. Click the OK button to close the Customize panel
8. Hit the shortcut key combination you just added
 
Expected: LibreOffice About page should be displayed (or what your selected function should supposed to do)
 
Actual: Nothing happens.

***
 
Also tried in Mac M1, kindly see the part with "/ Mac Arm64".

Version: 7.3.4.2 / LibreOffice Community
Build ID: 728fec16bd5f605073805c3c9e7c4212a0120dc5
CPU threads: 8; OS: Mac OS X 12.0.1; UI render: default; VCL: osx
Locale: en-PH (en_PH.UTF-8); UI: en-US
Calc: threaded


Not working
shift + alt + . / Mac Arm64 = “¯”
shift + alt + , / Mac Arm64 = “˘”
ctrl + shift + , / Mac Arm64 = “<”
ctrl + shift + . / Mac Arm64 = “>”
shift + ctrl + . / Mac Arm64 = “>”
shift + ctrl + , / Mac Arm64 = “<”
shift + ctrl + alt + ,  / Mac Arm64 = “¯”
shift + ctrl + alt + . / Mac Arm64 = “˘”
ctrl + alt + f1
shift + alt + ctrl + end
ctrl + alt + 0
ctrl + alt + f5

Working
ctrl + ,
ctrl + .
alt + 0
shift + ctrl + alt + ‘ 
shift + f10
ctrl + 8 
shift + = (follows normal key entry for both left and right keyboard keys)
ctrl + alt + [
ctrl + alt + z
shift + space
ctrl + alt + insert
shift + ctrl + alt + i
shift + alt + [
ctrl + = 
alt + a (opens Table menu of LO)
ctrl + space
shift + alt + escape
alt + 6 working
ctrl + . working
alt + . working
alt + , working

Looking at the sample shortcuts above, it seems that not all are working as expected (set behavior using Tools > Customize > Keyboard).

I also have seen similar bugs reported to Bugzilla or outside Bugzilla. You can check them using the links provided below.

**Related issues**

Similar bug report

some keyboards shortcuts not working - English - Ask LibreOffice (https://ask.libreoffice.org/t/some-keyboards-shortcuts-not-working/60626)
Custom keyboard shortcut not working - English - Ask LibreOffice (https://ask.libreoffice.org/t/custom-keyboard-shortcut-not-working/3810)
#974345 - libreoffice-writer: shortcut keys don't work - Debian Bug report logs (https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=974345)
[Libreoffice-bugs] [Bug 34804] New: Keyboard shortcut for Highlighting no longer works (https://libreoffice-bugs.freedesktop.narkive.com/qWfE5c8F/bug-34804-new-keyboard-shortcut-for-highlighting-no-longer-works)

Related issues outside Bugzilla
145298 – Keyboard Shortcuts partially unassignable with LibreOffice Writer (https://bugs.documentfoundation.org/show_bug.cgi?id=145298)

Replicated on 
Version: 7.3.4.2 (x64) / LibreOffice Community
Build ID: 728fec16bd5f605073805c3c9e7c4212a0120dc5
CPU threads: 4; OS: Windows 10.0 Build 19044; UI render: Skia/Raster; VCL: win
Locale: en-PH (en_PH); UI: en-US
Calc: CL

147996 – keyboardshortcut font-color doesn't work (https://bugs.documentfoundation.org/show_bug.cgi?id=147996)

(In reply to Esmeraldo from comment #5)
> Able to replicate the issue in the above versions.
=> Status NEW

*** This bug has been marked as a duplicate of bug 115052 ***