I'm been trying to figure out possible root causes for the OKD by going thru the source of the bootloader. Tho I'm not getting any leads, I'm reminded of a thread (link) I read quite some time back, that I also encountered with my Linksys EA-7500v2.
It could be that the change of moving FIP into UBI may have hit the same issue as discussed in the thread I linked above. I would think that the UBI IO driver implemented in BL2 is probably not as robust as those in the Linux kernel, and is not able to catch bit-flips? when loading from the MTD device?