I have a note 20 5g and for some reason my SMS app does not use the selected default sim card selected in settings. This happens on my old numbers, that I already had on my old phone. They use sim2, my work phone even if i have sim1, my personal sim selected as primary. When i change the sim in message settings it works only until i close the app then it goes to sim2 again. With the new numbers, that I introduced after I changed the phone it works just fine. The ones I had does not.
This happens for for messages only, calls and data is just fine.
Also, everything is up to date so there should be no problem there and I have al the necessary carrier settings installed on my phone.