What message app are you using? I have group chats with iPhone users that are fine, using the stock app from Google.
Discussion
using google messages yeah, it just stopped working recently.. (or started working worse).
Try to attach yourself to as less Google products as possible. It's an evil company not carrying about their users. This site speeks for itself
Check under message settings/advanced. Make sure group messaging is set to MMS not mass text. Other than that I'm not sure.