Version: 126.96.36.199 / LibreOffice Community
Build ID: 47f78053abe362b9384784d31a6e56f8511eb1c1
CPU threads: 4; OS: Linux 5.4; UI render: default; VCL: gtk3
Locale: en-GB (en_GB.UTF-8); UI: en-GB
This concerns the AppImage version on Linux Mint. There are issues with using menu and dialog shortcuts in LINUX generally. I will focus on the Find & replace dialog as an example.
Open the attached Writer file in a Linux distro. Open the Find & Replace dialog.
EXPECTED RESULT: The shortcut keys should be automatically underlined in the dialog.
ACTUAL RESULT: No letters are underlined; and, in fact, there is no shortcut letter underlining visible in the program AT ALL. You need to press Alt in order to bring up the shortcut letter underlining.
I am a new Linux user, but in Windows the shortcuts are automatically underlined. And in Windows dialogs you only have to press the letter to activate the shortcut – NOT the Alt button as well.
Note: Ignore the reference to an attached Writer file: just open any document.
Actually beleive this is expected behavior with GNOME GTK3 based desktop environment.
Implemented at 5.1 for bug 92630 
The Alt + <shortcut> behaviour is fine when accessing MENU commands. But in a dialog box, such as Find & replace, having to press TWO keys, i.e. Alt + <shortcut letter>, is a minus factor.
(In reply to V Stuart Foote from comment #2)
> Actually beleive this is expected behavior with GNOME GTK3 based desktop
> => NAB
I see the same thing in GIMP and Inkscape on Ubuntu 20.04/Cinnamon desktop. Since it matches the behaviour of the chosen OS interface, NOTABUG.