Mac Studio defaults to showing log in on startup only through the HDMI port, even when the main monitor is selected as Thunderbolt.

Quick update and perhaps the answer to my own question.

My computer has a two user setup, an administrator plus another user without privileges.

I never use the administrator account; it is there just for safety. So all the changes to the display arrangement were being made only to my usual account.

A few days ago, I tried applying the display arrangement to the administrator account, and it seems that the problem is solved! Now the login screen shows up on the preferred display.

So it seems to be a MacOS quirk after all. All accounts (or at least the administrator account) have to set the same display as the main one for the login window to appear there.

Tested and yes, it seems that it is a hardware behavior. No matter what screen is selected as primary, the startup login always goes to the HDMI port first. Hopefully future versions of MacOS will address this. Thanks!

Correct!

It seems that, at least for my particular setup, after rebooting, the user login screen only appears on the display that the administrator account has set up as the main one. The arrangement chosen by the non-admin account makes no difference.