Multi-recipient SMS Failures

This bug has been inconsistent over many years.

I am on iOS 13.5 on an iPhone 6s Plus on Mint.

Sometimes I can send a txt message (SMS since the recipients are both on android), sometimes the message fails.

Same people, same group. In my most recent conversation I sent a message to the group last night and it went through fine, and today it fails.

If this isn't a bug, I don't know what is. If it "isn't an apple problem" then does that mean that EVERY android phone also has problems with inconsistent ability to do multi-recipient SMS messages?

Would my carrier be responsible for SMS messages failing, to the point where it works one day and not the other?

Could my friends have changed their phones overnight so that they do not work the next morning?