After we flush ours, there's a few seconds of more water trickling down. I think it's designed into the system, so there's some water in it to seal the ball valve. That helps seal out odors from the black tank. Ours leaves about 1/2" to 3/4" of water on top of the ball valve. If it doesn't hold some water in it, the ball seal is leaking by, and either needs to be cleaned underneath, or be replaced.
Every day's a learning experience, though sometimes not what we expected.
2012 Jay Flight 19RD
2016 Ford F150 XLT 2X4 SC 3.5L Eco Max Tow
2010 Tundra TRD DBL Cab (Traded)
1 Spoiled Dog