iphone doesn't respect "conversation line" or "preferred line" when sending messages

There is a difference between "Last Used" and "Always Use"

if you have selected a Line for a Contact, Say Line B as "Always Use"

and If that contact sends you a text using Line A then if you reply to that contact using the received message in the Message App as a continuation of the conversation then the Contact will be assigned as "Last Used"

Always start a conversation from the Contacts App if you want to keep Business and Personal lines separate.

If you start the conversation from the old thread then you cannot change it. Start a new conversation from Contacts App. Here you can change the number as well as the Line. As shown in the GIF below

If you start the conversation from the old thread then you cannot change it. Start a new conversation from Contacts. Here you can change the number as well as the Line. As shown in the GIF below

Yes I have this problem too! It keeps reverting to my eSIM # for the conversation line and it won’t allow me to hold on my other selection.