This same thing happens to me when using iMessage on my MBP (no issues from iPhone). It's been happening for over a year now. I started noticing it after getting my 2021 14" M1 MBP, around the time of Ventura. This did not happen with my prior MBP running Monterey.
I'm dying for a solution. I send out different text threads constantly for work and I always experience the behavior you describe.
It's not really a "fix", but I have noticed that if you follow up immediately with a second text, all recipients (including SMS Android) will see it as a group message and all subsequent communication occurs on a group thread as intended. So you could start with, "Hey everyone" as the first text, and then send the important stuff as the second text. Very annoying, but at least gets everyone on the same page quickly.
IdrisSeabright - It's behaving more like an OS issue/bug than something to just turn off and turn on again (mainly because it seems to work as described once two messages are sent from the original sender). I know this is a possible help, and may try if I ever am able to get all my devices in one location (a challenge in and of itself).