Your pump is pulling liquid faster than it will drain through the bed by gravity alone.
The typical tube style mash screen does not provide enough surface area
for a fast flow rate even with a thin mash.
From other threads, this seems to be less of a problem with a false bottom.
Once the grain bed settles, flow rate decreases significantly.
I suppose the pump could also be compacting the grain bed
by creating lower pressure at the bottom of the bed.
Which would make the situation worse of course.
We seal our tubing, connections, pump housing to keep liquid in.
It might be too much to expect them to keep air out under increasing negative pressure.
The solution is to slow down the recirculation rate to the rate of normal gravity drain.
But I don't know how you would do that without a pressure meter before the pump.
I gravity drain from my mlt to the bk and then drain to the pump which recircs back to the mlt.
I have to slow down my pump to match the mlt drain or the pump runs dry.
I never get pump cavitation during mash recirculation.
The brutus 10 folks don't seem to have this problem.
I suspect a false bottom on a 15 gal pot with a thin mash gives them
a good enough drain rate that the pump can run fast.
I am interested in how the false bottom works for you.
I may give that a try in my round cooler to increase my mlt drain rate.
(most of the above is opinion based on experience on my system, ymmv)