Standard issue on blocked domain hunting: How do I determine what domain an ad is coming from?
But I'd also recommend to take a closer look at your gravity output on update: You could have been using some incompatible blocklist ever since v4 (but you should not be using blocklists that are not meant to be used by Pi-hole in the first place).
As Pi-hole 5.0 puts in extra effort to solicit valid domains from files, you may now end up with an unexpected blocked domain.
For each blocklist imported during gravity update, Pi-hole 5.0 will notify you of invalid content.
Check whether that may be the case for you.