Armbian 22.02 release announcement

[Timpanogos Slim]

That, for sure, is a deficiency in uboot, not to tell version -clearly- somewhere
That it WILL boot with emmc enabled and NOT boot with it disabled strongly
suggest that the uboots on emmc and uSD differ
(but not for sure, it could be the OS doesn't drive the display properly, ie booted but no video)
Your last para is not so clear, maybe manjaro boots and can run x11? (but no serial uboot comms)??
But with uSD (armbian) no x11? (maybe try a different dm - display manager. gdm, xdm, sddm, lightdm etc)

With the current manjaro i get either a text console over serial or x11 on the display. 

My PBP was purchased almost exactly 2 years ago and i vaguely recall installing a uboot update to enable some better sleep mode or something.