Incoming email replies not appearing on document timeline

I’ve had this working in the past, but at some point things seem to have changed. I’m on v13-beta5 and using gmail as my email provider.

When I send emails from the timeline of a document, the outgoing email sends correctly and gets appended to the document’s timeline.

If the recipient of that email replies, however, the reply shows up in the communications inbox but does not get linked to the originating document.

Is anyone able to get incoming replies to link to the timeline?

Any ideas on this? Am I misunderstanding the intended functionality?

Am I completely misremembering how this functionality worked?