Smells like a browser issue to me, especially since AFAIK Opera Mobile uses Blink/Chromium under the hood which does not seem to have this problem. Is this a new problem or has it been like this for a while? It also looks like Opera Mobile 82.3 was released today so it might be worth testing again with that.
Updated my Opera to 82.3. The issue is still there
It seems it is a new problem as I visit XF almost every day and was offline maybe for two days, then came back and noticed it about two days ago. The issue was not there about a week ago.
I remember I experienced unexpected browsers behaviour to things like css wrapping and css breaking text. Maybe it is something similar.