Same sitch, it happens off and on. I’m on Android, most of my extended family is on iPhone (except my brother). Our group texts are generally fine, but like 10% of the time these weird things happen where messages are out of order, or don’t go through at all.
I think as others have said it’s just SMS/MMS vs. iMessage, and until there’s any semblance of a unified protocol, this is what we have to deal with.
Mostly it’s just a nuisance, but missing picking someone up at the airport because of it one time was not a good thing.