As was mentioned in the other thread you have there is always some fluctuation with a Herbie drain.
Possibly due to barometric pressures. One thing we can not see is how far below water level is the main drain in the sump. No more than 1.5 inches or back pressure affects how the pipe drains. The closer to the top the better, in fact 1 of mine is @ water level with a 45 degree fitting.