It seems that starting from 6.3 series, the "Edit Fields" dialog (first create a field with Insert -> Field -> Date, then select the field and Edit -> Fields to open the dialog) added mnemonic keys to the buttons at the bottom (Help, OK, and Cancel). I'm guessing it started in 6.3 series because in 6.2.8 they don't have mnemonic keys, and in 6.3.4 they do.
The "OK" button uses letter "O" for the mnemonic, however it clashes with the "Format" column that's already using "O". Now when I press Alt+O the focus goes to the OK button instead of the Format column as it used to. Since the "Format" column used "O" first, and it also exists in the Insert -> Field -> More Fields... dialog and maybe others, the "OK" button probably should yield and use "K" instead.
Similarly, in the same dialog, the "Select" column, the "Offset in days" spinbox, and the "Edit" button all use "E" as mnemonic. But that's probably less problematic to users as those are less frequently accessed from keyboard.
My "About LO" information:
Version: 18.104.22.168 (x64)
Build ID: 1b6477b31f0334bd8620a96f0aeeb449b587be9f
CPU threads: 2; OS: Windows 10.0 Build 18363; UI render: default; VCL: win;
Locale: zh-CN (zh_CN); UI-Language: en-US
I confirm it with
Version: 22.214.171.124.alpha0+ (x64)
Build ID: e26d89371f0e4f41476c9a99be01d98dedb76776
CPU threads: 4; OS: Windows 10.0 Build 18362; UI render: default; VCL: win;
Locale: de-DE (de_DE); UI-Language: en-GB