MediaWiki applies CSS classes to the individual rows in the watchlist to indicate whether the user has actually viewed that change (e.g. visited the page and seen a revision including the change). These classes are somewhat-unintuitively named mw-changeslist-line-watched for unviewed changes and mw-changeslist-line-not-watched for viewed changes.
Flow topic changes included in the watchlist are always using the mw-changeslist-line-not-watched class, even when the topic has not been viewed.
[Update]: Upon further testing, "top-level" replies to a topic aren't highlighting, but nested replies are. There's also no indication of which "top-level" replies haven't been viewed yet when viewing the thread.
Every line in this (screenshot) should be class=mw-changeslist-line-watched until I have visited the most recent version.
Similarly in Recent Changes, where that class is applied to my watchlisted pages. Eg. in screenshot, both [[ResourceLoader]] and the Flow Topic name should both be bolded, because they have new changes since the last time I visited the page:
