So far, I’ve not been able to reproduce the issue. On a test site, I created a group chat with three other users, (0 admins, 1 moderator, 2 normal users). I sent a handful of messages, and ~3 minutes later, all three users got their emails.
None of those test users had visited the site for quite some time.
Can you try creating a non-admin test user on your site using a + address to test this out and see if you can reproduce it?