I too have had this exact same problem since buying a Windows 11 Home laptop in Dec 2024. My mobile is Android 11, never had this problem before. Has sent me crazy, have tried too many things to mention.
Long story short, yesterday morning I installed Phone Link version 1.24111.82.0 from this site Download Phone Link 1.24111.82.0 for Windows | Uptodown.com on my laptop and I've had no problems since!
Previous laptop version was 1.24112.110.0
Current mobile phone version is 1.24112.138.0, I haven't changed that.
Phone link on your pc will still show it being a group chat but hopefully it won't create multiple texts like it was doing and your phone should show it as being the one contact.
Hope this sorts it for you!
If not, I'd look at installing other older versions of phone link until you find the one that works for you.