@Georg I wouldn't rely on a series' "ambience" to determine if something like NTR is possible or not, because an author can easily jump the shark when they run out of ideas; while I didn't read it myself, a fine example is what I've heard of Gal Cleaning: started out as a lighthearted series about a series bookworm helping a gyaru keep her room clean and healthy that took a sharp downward turn when it started throwing rape as drama into it.
But really, most people cry "NTR bait" when stuff like this happens because, in a way, it is. The bait isn't that the thing actually happens, but there's the implication of it happening; like the series Hibike Euphorium (another I didn't experience personally, just heard the fallout), which apparently had heavy implications of yuri romance in it, only for the author to chicken out at the last moment, so fans derided it as being yuri bait. Following that logic, NTR bait would, then, be any character put in a romance series for the sole purpose of disrupting the series' focal relationship and creating drama between the characters. If said character
actually forcefully intruded on the relationship and stole one of the protagonists away, it would no longer be "bait", but actual NTR.
@gravy141 Conflict is indeed necessary to an interesting story; any writer knows that as an absolute fact. The difference is in the value of the conflict. Whenever a romance series does this, it's
incredibly easy to predict how things are going to go: the ex will interact with their former love interest in a way that sparks insecurity in the other protagonist, whether intentionally or not. The love interest that was involved generally won't reciprocate at all, but they'll be confused by their ex suddenly showing up and reinserting themselves back into their life; this leads to situations that can be easily misunderstood as the love interest flirting with their ex, reinforcing the insecurity of the other protagonist. Eventually, however, if the situation was unintentional, the ex will back off and support the main couple after clearing up the misunderstandings. If, on the other hand, the situation was intentional (which is rarer, in my experience), it leads to more dragged out conflicts that don't do the story any favors, because the writer often backs themselves into a corner by not having a good solution prepared, occasionally having to resort to drastic measures to resolve things or outright retconning certain actions of the ex.