Just ran into this, and eventually, I got it working but with some jigging and poking.
So to test, first start by doing a clean start
Fullscreen Screen 1 on Display 1
Fullscreen Screen 2 on Display 2
Exit and Safe
Restart and Display 2 goes full screen onto Display 1 not 2 as expected.
If you don't maximize the window it does remember both the position and display. This led me to realize if I do the following It seems to remember where it should be in fullscreen.
Fullscreen Screen 1 on Display 1
Place Screen 2 on Display 2 (NOT in fullscreen)
Exit and Save
Restart
Fullscreen Display 2 on screen 2
Exit and Save
Restart and Now Screen 2 is full screen on Display 2
Hopefully, this survives a reboot.