I've got a Jabsco Electric head
and have never had that problem.
I did however, 'forget' to close the intake seacock but to my relief about an hour later (after rushing back to the marina to check ) "did I close that pesky seacock + the flush seacock" but all was ok
Glad I went back because both intake and exit seacocks were not closed.. checked the toilet and normal water level.
Here's my setup as a reference, the loop is clearly visible.
Yes, my head
is mounted below the water line