system reverted to Ubuntu after Proton VPN install

[stevef]

complete reversion to the Ubuntu side of things

kinda almost like a fresh install, honestly.

This indicates something more is wrong. I recall another report that a Linux Lite installation had lost the LL modifications.
It was maybe a couple of years ago and I can't find that thread to compare with your experience but if I recall correctly it wasn't established what had happened.

In your position, I'd wait to see if Jerry/Valtam or Trinidad have any insight.