Is it a red flag for a guy to be “best friends” with his previous girlfriend?

First of I have a lot of friends of male friends and there’s no issue with that. Anyway, I am 42 F talking to a 50 M from Hinge and we have a lot in common, similar values and interests, and great conversation, but I just found out he’s best friends…not friends but “best friends” with his ex from a couple years ago. Since they’re not coparenting and not just in the same friend group this seems weird to me? And they were hanging out watching a movie last night? Is this not weird? Need any thoughts, advice, opinions because I am ready to say “nice knowing you”. (This post is NOT about a breakup I am sick of these bots flagging everything.)

TL:DR this is about a guy being besties with a woman he once dated and if this is an issue