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)
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)