Spontaneous numlock

[dsimic]

That kernel is rather old.  Manjaro ARM is currently at 5.11.11 (but still ships 5.11.6), so the kernel on your Debian should be updated before doing any further debugging.  Unfortunately, my experience with Debian is very limited, so I'm unable to provide further advice on how to actually do that.  The easiest and quickest way to go forward with the debugging should be to boot Manjaro ARM from a microSD card, test the "Fn" keys, and check if the original issue is still there.

Based on your description, either something is really wrong with the keyboard in your PineBook Pro (which hopefully isn't the case) or something is wrong/missing in the variant of Debian you're running.  On my PineBook Pro, which is running Manjaro ARM, there are no issues with the "Fn" keys; the screen brightness keys, speaker volume keys, NumLock, CapLock, etc. all work as expected out of the box.

Edit: Based on my limited insight, it seems that the configured APT kernel repository points to a custom repository that contains no kernel packages newer than 5.7.0.  I have no idea if there is a repository that offers newer versions of the kernel packages.  Also, please have a look at this wiki page, which contains instructions on how to resolve some of the issues you're experiencing.