There is a software conflict between Logi Options+ and Bitwarden (password manager) on macOS Big Sur 11.5.2.

When Bitwarden is running (not the active app), custom settings and some basic functionality of both the MX Keys and MX Mouse fail.

While most of the basic functionality works things like custom buttons, back & forward buttons, scrolling, F-Keys, and others just don't work. It's as if the Logi Options+ software just doesn't know the hardware is there. In fact, Logi Options+ typically will just indicate both the keyboard and mouse as "inactive."

Quitting Bitwarden software, setting it to not start at launch, and re-logging-in fixes the problem.