And as a reply from Microsoft to this particular post this is what I received, more than once I must say, dated back to December 2023, a copy/paste from the looks of it. It is amazing that they did not find a solution yet since Dec 2023...
Hi BBS10,
A reply to a question you are following on Microsoft Support Community was unmarked as an answer.
The Outlook Team did some work in late December to address this issue. I am not sure the fix build though. I just tested this with my version at work with my Gmail account and I was able to see it working properly. I did read/unread in New Outlook and it showed up on Gmail web ~10-20 seconds later.
In New Outlook if I go to Help, Diagnostics, my version shows as: Client Version: 20240112004.07.
If you are at this version or higher and still repro it would probably be worth submitting an In App ticket (Help, Support).