I got lucky on google! Turns out, it was Cisco Webex hogging ctrl-shift-M, for "music mode" (no idea what that is). So Geany and Firefox couldn't see it (unless run as admin). Still, no idea why Chrome was able to see it (best guess is, this is a company machine, and the company might have a special build of Chrome that runs on a higher orbit).
Force-closing webex fixed it. The webex processes kept coming back as I was killing them (so the issue would be fixed for a few seconds, then come back). Some parent PID was respawning them. The powershell-command I ended up running to perma-kill webex was
`taskkill /F /IM ciscowebexstart.exe /T`
And it works now!
Gonna leave this here in case someone else runs into the same problem
https://www.tenforums.com/microsoft-office-365/148737-outlook-hotkey-ctrl-sh...