The issue I'm having is essentially the same as the issue in this 9+ year old DayZ bug report: https://feedback.bistudio.com/T104003
In my case, I am using a Redragon DITI Mechanical Gaming Keyboard.
To solve the issue, I tried reinstalling Arma Reforger, erasing all my mods and trying again on the vanilla game, and shutting down all non-essential startup tasks in Windows, in case some other app was interfering. This issue only happens on Arma Reforger. More system info:
Operating System: Windows 11 Home
Processor: 11th Gen Intel(R) Core(TM) i7-11700F @ 2.50GHz 2.50 GHz
Installed RAM: 32.0 GB (31.7 GB usable)
This same issue also occurs in the Experimental build of Arma Reforger.