See the layout where there's an additional tank on the ground floor of the tank setup? That seems to cause the xp bar to stop depleting when you step onto it (the xp bar stops at just over 3 of the segments in the bar away from 0xp). Removing that tank block and stepping back onto the drain depletes it entirely, as expected.

When the xp bar stops depleting with the extra tank present, xp is continually added to the tank setup, essentially forever.

Version details:

Direwolf20 Pack version 1.0.14
Openblocks version 1.2.4-snapshot-151

It was (maybe) fixed few days ago. Fix already in recent version.