imessage not using default line as set in the contact

I have 2 lines (business & personal) on my iPhone 13. I set default lines to use in each contact. When I send an imessage to a contact that has the default set to use the business line, I am often frustrated to find that the message got sent from my personal line. Sometimes, even when I change the line to business during the imessage conversation, it still reverts back to my personal line. I don't have this problem with phone calls, only with imessage. Why does it do that???