System disrespects preselected SIM card choice

I am using my phone (Berrylium, 0.7-o-20190925) with 2 SIM cards (X and Y). Y is preset as my standard choice to send out text messages.

Since a little while (unable to be more precise, sorry) I notified that when I am sending messages to people I do not have an open conversation with, SIM X is used as standard setting.

How it should be: I may chose under SETTINGS > NETWORK & INTERNET > SIM CARDS my preferred SIM card for mobile, data, calls and text. It would be desirable if this choice is being memorized and respected by the system.