It is not, the delay exists, I did some test for different purpose 2 days ago. But I think it's a good thing (the delay). It kinda reinforces the idea that you should build more than 1 factory (otherwise, what is their purpose? - just a waste of mass). Not to mention you get 0 adjacency bonus effects for the assisting units - so you could in theory get 0% bonus if your assisting BP is overwhelmingly larger than the factory's. Say with the case of a bunch of HIVES assisting a T3 Air factory - you capping it with T3 PGens is effectively useless at that point.
What worries me though, is the delay between a factory finishing it's upgrade and starting working on new units. I sometimes feel like it somehow gets stuck, and you have to wait for an extended period of time before it starts making units from the queue. And sometimes, though rarely, and I'm not 100% but pretty sure, if you queue a unit in that post-upgrade blackout, the unit gets immediately de-queued and you have to press for it again (assuming the queue after the upgrade was empty).