iPhone merging messages

I’m having an issue on my iphone se 2020 where my husband’s iMessage thread suddenly became unpinned and disappeared from my messages. If I create a new message and put in his contact or even type his name, it automatically jumps to a group text between my sister and my mother, and they get the message instead of him. If he messages me, it shows up in the group text with my mother and sister. All four of us have different apple ID’s and do not share anything in common in our contacts. I can call him just fine, and no other contacts are impacted. I have deleted the all the contacts, updated, restored, turned off iMessage, but nothing has worked. If I send him a text using his number, but no area code, it sends to him, but as soon as he replies it goes back to the group text. If I put my sim card in another iPhone, everything works properly. Is there any fix aside from a new phone?