This WU has been returned by you and one other person. Both had EUEs but at different percent-complete.
The guarded-run exceptions are a known problem which is getting a lot of attention by the development team. There's no way to know when a software update will be available to improve or fix this sort of situation.
Aha. See, if the client collected & sent hardware info when it returned a not-quite-normal WU result, we'd have a much better idea if it's hardware limitations (as it probably is in my case - I suspect that my GPU simply doesn't have enough shaders or enough memory bandwidth) or a software bug (as seemed to be case when the 1.13 & 1.15 cores came out)
(And I was just expecting toTOW to give his usual "someone else completed it"...)
The number of shaders will not determine which WUs will run and which will not. It determines speed. It's most easily understood if the shaders are actually creating an image on the screen rather than folding, but the same concept applies. If your screen resolution is X by Y, then there are X(times)Y individual pixels that must be evaluated. If one is passed to each shader it won't finish the screen so a second batch of pixels waits behind the first batch and the process continues until every pixel has been processed by a shader. Upgrade your card to more shaders, and each shader has to processs fewer pixels. Similarly, more memory bandwidth may make the frame rate of your games higher but it's still only a speed difference, not a difference that causes WUs to fail.
I vote for a software bug, but it could also be a hardware problem. If new software fixes it, then we'll know for sure (whenever that happens).