I'm writing this on behalf of my brother, the actual S21 owner, because I am the one affected by this issue and also the one who understands what's not working as intended.
My primary phone number is a Google Voice number, and that usually means I get texts sent to me in the Google Voice app. When my brother texts me, as of today, his Samsung Messages app shows that it's sending to my Google number, but I receive the message at another number that I've never given him (the "real" carrier number attached to my phone).
I've personally verified that my carrier number is not in my contact entry on his S21, and the Messages conversation shows my Google number as expected if my contact is deleted. (That was one step we tried to fix this.)
If there is some possible way that Samsung Messages has linked my numbers together, I'd like to know more about it and see if it's possible to unlink them.
I've also posted this problem in the Google Voice help community, where we eliminated the obvious possibilities that have to do with the Google side of this.
Device settings>your profile picture (next to settings search icon)>account information and my information and check the phone number listed.
I didn't have a Samsung account until shortly before posting this thread, haha. My own phone is an ASUS device. This is a good theory, though! That chat feature is exactly the kind of thing I'm looking to learn about in case it could be causing this.