iPhone XS combining two imessage threads into one

I just got a brand new iPhone XS. I noticed an issue yesterday where two iMessage conversations sent to separate phone numbers are combining into one thread. On my iPad, they are still being stored as separate conversations. I think this is a bug.

Here's what I've done so far:

-Deleted the conversations from my iPhone and iPad

-Deleted and re-added the contacts

-Reset my phone to default settings.

After performing the above steps, I started a new conversation thread from my phone to each contact. They stayed separate until both answered, but then the conversations combined again. When I try to send a message to one of the contacts, it sends it to the other on the combined thread. The only thing that these two numbers have in common is that I believe they use the same apple id, but I have had separate threads for them for years and they are still separate on my iPad.

Please help!