Is desktop Messages messing up anyone else's texting groups?

I've been having weird problems with Messages and texting groups. If I post in one group, I get replies from friends in another group. I've tried removing groups with similar people in them, but that just adds to the confusion. It's also adding people to similar groups.

I'm seeing if not using Messages on my MacBook Pro stops these issues from happening. I suspect it's the culprit.

On a maybe unrelated note, Calendar keeps changing to Monday being the first day of the week. I have it set to Sunday. When it does that, I go into settings and it says Sunday is the first day of the week. I then change it to Monday, then back to Sunday and then it stays on Sunday for a while.

Yes, I keep everything up to date, so I'm on the latest versions.