So it's a motorized ball valve controlled by software. It takes approximately 5 seconds to fully open, so there shouldn't be much vacuum.
Also, I poked a pipe cleaner in through the ball valve (disconnected all tubing) and liquid started to come out. I took the pipe cleaner out... And the liquid flow stopped. As I mentioned, I couldn't even suck the liquid out by mouth like a straw. You're correct, it was completely clogged immediately upon starting vorlauf, so I never got any sort of first (or any) runnings.
The only thing I can think of is that slight warping of the false bottom. Its not extreme, but it's definitely enough for a few grains here and there... Which is enough to cause trouble