Yes. It's because the Messages UI conflates iMessage groups (blue bubbles) with SMS/MMS/RCS groups (green bubbles), combined with the fact that you can iMessage

Yes. It's because the Messages UI conflates iMessage groups (blue bubbles) with SMS/MMS/RCS groups (green bubbles), combined with the fact that you can iMessage an Apple user using either their phone number or any email address they have enabled for iMessaging. So say you start a group message to Apple users. You add type their names and don't pay attention to whether Messages is using their phone number or email address to add them to the group. (Even worse, after you're done typing their name, Messages only shows the name so you can't even tell how they've been added w/o taping their name again.) Now, when it's an iMessage group (blue bubbles), it doesn't matter how you added them. But as soon as you add a non-Apple user, it becomes an SMS/MMS/RCS group (green bubbles). Guess what happens to the Apple users who were added by email address? That's right, they get emails. There's no indication whatsoever to anyone in the group that this is happening. And as a member of the group, you can't fix how you were added. The message group has to be abandoned and a new group needs to be created using only phone numbers. It's a terrible UI. I've been dealing with it for years because originally I used Google Voice and there's both Apple and Android users in my extended family. But even after I finally ported my number over to my phone, my extended family still sometimes adds me to SMS/MMS/RCS groups using my email address. (Why don't we just use WhatsApp you ask? Oh, we use that too. With varying degrees of technical literacy, we end up using all the things.)