macOS Mojave confuses displays at boot

Using a Mac mini 2014 with two mini DisplayPort (mDP) to DisplayPort cables with two Dell P2415Q monitors, or a Mac mini 2018 with StarTech Mini DisplayPort to USB-C adapters for the same cables, macOS Mojave 10.14.1 is unable to distinguish between the monitors at boot.

Since one monitor (primary) is horizontal (landscape) and the other is vertical (portrait), this means the login screen may appear as portrait mode (sideways) on the horizontal display, or as horizontal mode (sideways) on the portrait display, or sometimes for variety in portrait mode on the vertical display. But only apparently by chance as landscape mode on the primary horizontal display where it is supposed to be. This makes logging in challenging.

The display orientations and primary versus secondary fix themselves after login, so clearly Mojave can tell which monitor is which after login.

Apple: Please fix this bug. I'm tired of turning my head sideways to log in.