OK I think I have narrowed down the issue.
It appears to be associated with FireFox when the zoom is set at 120%.
I went backwards and forwards with one user experiencing the issue. Trying different browsers, devices etc. In short using Firefox Nightly (91.0a1 (2021-05-31) (64-bit)) at 120% zoom it causes links to not be resized.
If the zoom is adjusted back down to 110% or 100% the posts are resized perfectly.
Odd I know but here’s the thread: