It sounds like the UAT is under a lot of suction due to restriction in the fuel system (it includes everything ... from the vent all the way to the pump suction nipple).
When you cut the power and the pump voltage drops, the pump may not be able continue sucking fuel at that voltage for a short period of time ... until the pressure from the vent builds up again in the system.
In my experience, rigid UAT's are much more susceptible to cause your issue.
If your UAT isn't rigid, check how much it collapses at high power. That will give you an indication of the amount of restriction.


Jack