Pumps,
Just upgraded here from 2018 to 2019 (610)

Using dual 4K TVs, and the best example I would give anyone, is using DaVinci Resolve video editor, multiple monitors is implemented superbly!
My left side monitor is the Primary, right side is the secondary, during bootup, BIOS appears on the Primary, then Win10 is extended to the secondary.

Using Win10 (1803 version), Nvidia GTX-970, BIAB 2019(610) native resolution is 3840x2160, "Scale and Layout" is set to 150%.

The 2018 version was working just fine, displaying full screen on either monitor, although as you referred to, the splash screen, the green and yellow messages, options, etc. would by default always display on the Primary screen, even if BIAB was full screen on the secondary. That is true of most programs being used on dual monitors, and I guess I'm just used to it. The 2019 version works the same way, I don't see any change there.

Jerry
2019 BiaB Megapack (634), RB 2019 (Build 4)
Reaper 5.983 x64, Audient id14 USB interface, Voicemeeter Potato 3.0.0.8, Nektar LX49+, Roland E20
i7-6700K Skylake desktop, 64bit Win10 Pro(1903), 32GB ram, 512GB system SSD + 2TB data SSD