Installed Karabiner, now getting § instead of tilde/backtick

Happy New Year everyone! MBP 14 M3 Pro with all recent updates. US English layout. This all started because I wanted to map LOpt + ijkl to arrow keys so that I didn't have to move my right hand from the home row to navigate. I want to set this for the built-in keyboard *and* for my external Qwertykeys QK100. I did a little research which pointed me to Karabiner-Elements, so I installed that via brew. Not only have I not figured out how to configure Karabiner properly, but ever since, my external keyboard's tilde/backtick key has been producing §/± instead, in spite of all visible System Settings being US English/ANSI. The internal keyboard works fine and produces `/~. I even uninstalled Karabiner, since that was when the problem started, however, I'm still stuck on §/±. Can someone please help me restore my external keyboard's tilde key so I can get on with trying to figure out how to remap ijkl as optional arrow keys? EDIT: I've reinstalled Karabiner and tried to remap non_us_backslash to grave_accent_and_tilde to no avail. I still get §/±. I've also noticed that the keyboard only does this in Mac mode. I've ensured that the keyboard's software correctly maps the key to grave accent/tilde, however, switching back to "Win" mode resolves the problem. Unfortunately, it also deprives me of the correct modifier keys, so I'd like to keep Mac mode on. I've reached out to the mfct to see if they can help, but since it only occurs on MacOS I'm not sure if it's a problem on their side. EDIT2: My current workaround is to use the QK100 software to map `/~ to NUBS and then use Karabiner to map NUBS to grave accent/~. This has restored the normal functionality but I'd really like to know what the problem is.