Hey Jaffa,
I have the exact same issue on my Ugoos AM6 plus (not B).
(but B revision according to clock speeds based on Ugoos AM6Plus coreelec topic:
Ugoos AM6 Plus - #4 by boot2k3)
Did you figure anything out yet?
Thanks 
UPDATE: I figured it out! I let the long-not-updated Ugoos to update to the latest 0.5.4 firmware, and now it booted with am6 dtb on amlogic-ng-dv.
So I guess some changes in the firmware were needed for it.