I've been struggling with this for a few months now. I don't know why, but my Office installation keeps reverting to version 2205.
My real goal is to use Insider versions, but this issue affects me from even using current public release versions.
I've tried uninstalling and reinstalling Office. While I can successfully install, activate/sign-in, and run newer builds like 2206, 2207, 2208, and now 2209 --- anytime I do, I will receive an "update" within a few hours that reverts me backwards to 2205.
After that happens even if I try to turn on Insider versions, I'll remain forever stuck on version 2205.
Checking for updates simply tells me that no updates are available.
Since joining insider doesn't deliver me an update, the only way I've found to install insider versions (or indeed any newer version that 2205) is to use the Office Deployment Tool on my machine, which lets me run the new version for a few hours until it "updates" back to 2205.
I've even ran the Office Scrub program to try to remove all trace of Office from my machine, but yet this keeps recurring.
I have one other machine where it works perfectly normal. In fact it was on version 2208 even though it wasn't even using Insider.