Seems to be a software issue. I was getting this error too after activating my eSim on T-Mobile. After updating to the latest public beta, I haven't had this error at all. I haven't had any other issues with the current beta, just thankful that it addressed this issue.
If you are using the eSim then why do you have to eject the sim tray?
I have a sim card but have not used it yet but will do so. I have 10 GB free for 180 days so I will use it a bit later since I am still to take my iPad Pro out of my home. Waiting for the Smart Folio Keyboard case. Once I get it will then take it out.
Now with the eSim I can get another number like the dual sim iPhone Xs?
I am also on latest update public beta hoping it gets fixed with next update