Pretty normal I think,
The process of steaming will tend to empty a boiler, you dont hear the pump running therefore nothing is actually refilling it.
Under normal useage [eg 600 ml jug], steaming wont fully empty the boiler, but you will end up with some space eg 1/2 water and 1/2 steam.
Water is in-compressable, steam/ air is compressable.
What I think is happening is that when you hit the brew switch: the increased steam pressure assists in forcing water [and steam] out of the group.
At some point, once the steam pressure has run out, the pump has to do some "catch-up" work which is where you get less/no flow.
Always prime the boiler after steaming: you get the added benefit of quickly bringing the temp cycle down toward the brew temp.