Copying a mailing results in thousands of entries in civicrm_mailing_group, rendering page unresponsive

I'll start by saying I haven't been able to replicate this - but I've seen it happen multiple times on Civi 5.39 to 5.41. I'm hoping someone else finds this and it rings a bell.

Sometimes, when a client goes to edit a mailing, the page is either a) unresponsive, or b) doesn't allow saving. The "calculating recipients" SQL is running indefinitely. When investigating, there are tens of thousands of entries in civicrm_mailing_group for that mailing - the entity_ids repeat.

While I'm unable to replicate, this seems to happen when:

• Users copy an existing mailing rather than starting from a new mailing;
• In both cases, mailings were going out to 17-24 groups.