I've been using messages.android.com for a few weeks now and it's amazing, even if sluggish when "new conversation" is clicked.
The only issue I have is that if I'm texting more than one person it switches to sending as MMS and I can't seem to control it. I tried this but that setting doesn't seem to be listed. I'm using the default Messaging app on a dual sim moto g5. My phone recently upgraded itself to Android 8 but it hasn't solved the issue. It seems to happen, even with very short messages, and even if it's just text without any subject or media. The message converts to MMS as soon as I add a second recipient. Any suggestions? I will add a Windows steps recorder when I have a bit more time. If I add two recipients to a text message on the phone itself it works as normal and remains an SMS.
It would be useful if someone could see if they get the same behaviour.
Another edit: the problem persists with the new interface, whether opened directly or through a redirection from the old one.