Spotted at https://meta.wikimedia.org/wiki/Strategic_Wikimedia_Affiliates_Network on Meta. Despite having marked my watchlist as 'all changes seen' today, this diff from yesterday still appears in bold in my watchlist:
|mediawiki/core||master||+1 -1||Change style for class mw-history-suppressed from bold to grey (Base 10)|
The bold is not for seen/unseen, it was added for revision deleted (suppression/oversight) things by https://gerrit.wikimedia.org/r/c/mediawiki/core/+/678715 / T23272
Seems not useful for Special:Watchlist, because the same text gets bold from ".mw-changeslist-line-watched .mw-title" when unseen.
Checked on enwiki betalabs - the issue is confirmed: if the suppression option is clicked on Special:RevisionDelete, the bod will stay on Watchlist evn when "mark all changes as seen " will be clicked. in the screenshot below two entries were deleted. One was deleted without suppression- it changes to unbold when "Mark as seen"; when a revision was suppressed - the bold would stay.
Note: On RC the suppressed revisions will have double strike-out: