Battery saver turned off when opening Activity menu.

Identical issue here, Fenix 6 Pro (not solar)

Used to leave the watch in battery saver mode all the time but now it is disabled when opening the main menu. Only started since the 23 update.

I confirm this unexpected behaviour also on my Fenix 6 Pro (no solar).

I think it's a side effect of this fix listed in the 23.00 update: Fixed a potential issue where Battery Saver mode would randomly turn on after an activity or accessing a menu.

Any idea how to actually raise this as a bug with Garmin?

Same issue. 

It was already raised as a bug for 23.00 beta, but got no attention. Voting for the bug still works, but I don't know if it makes sense.

https://forums.garmin.com/beta-program/fenix-6-series/bug-reports/i/public-beta-23-00/battery-save-mode-turns-off-automatically

Apart from that, you can contact Garmin support with your bug report. Maybe Garmin - Chris can also help by forwarding your reports to the engineering team. 

I also have this error
Fenix 6 pro

Not fixed at FW 23.01

Any change detectable in FW 23.02 ?

Version 23.02 Beta, problem still exists. Fenix 6X.