If you do not have the YouTube app installed and you open a YouTube link there is no way to close the YouTube window that opens and return to mlem without closing mlem.
Device: ipad pro 11" (2018)
iOS: current, non-beta
Mlem: test flight
Added to our issue board, thanks for the bug report!
It appears to not be just YouTube links. I got locked into a link just now. I took a screenshot of what i see, i also was able to find the post again.
Unlike normally when you open a link there is no done button on the left and no safari icon on the right. The hide preview link does not help. Again i had to swipe away mlem to close it.
It does not appear to happen 100% of the time. I am trying to find the video that just caused it tonight (this is not the first time it has happened) but it was a misclick and am unsure exactly which link caused it.
I will post a YouTube link if i do find one causing it.