This might be an enhancement request, depending on your view. But since multiselection is common for anything related to drawing, I interpret this behaviour as a bug.
1. Create Frame
2. Anchor it "As Character"
3. Add 3 text boxes (write a single letter in each box)
4. Select one text box
5. Press and hold ctrl and left-click on another text box
Expected: Both text boxes become selected.
Actual: Only second text box becomes selected (selection of first text box vanishes).
I confirm it with
Version: 18.104.22.168.alpha0+ (x64)
Build ID: 460908269972fd1f89312a1e62897ed1503e9e98
CPU threads: 4; OS: Windows 10.0; UI render: default; VCL: win;
TinderBox: Win-x86_64@42, Branch:master, Time: 2019-09-30_09:18:03
Locale: de-DE (de_DE); UI-Language: en-US
and also with
Version: 22.214.171.124 (x64)
CPU-Threads: 4; BS: Windows 6.19; UI-Render: GL;
Gebietsschema: de-DE (de_DE); Calc: group
But I'm not sure, if this is a bug, an enhancement request or simply the expected result. => cc: Design Team
Additional information: Also text boxes outside a frame can't be multi-selected
In LibreOffice multi-selection is done with <Shift>+click; which works correctly for the use case of OP.
Dupe to bug 121967 for enhancement request to enable optional use of <Ctrl>+click for multi-selection.
*** This bug has been marked as a duplicate of bug 121967 ***