Hello everyone - interesting problem I hope someone can shed some light on.
My pool is set up for a propane heater but the heater is not there (bought house on foreclosure, they took it). When they took the heater they bridged the inlet and outlet pipes together to complete the loop. They did a shotty job and the pipe was leaking at the joint causing me to lose water. I cut off their connections and put new ones on, and now the leak is resolved - hooray! I have attached a picture of the leaking joint for reference. It looks the same now, except that it is lower (from cutting it) and it does not leak.
Since I made this repair, my hot tub is now draining in between pump cycles. The pool is gunite, in ground, kidney bean shape, and the hot tub sits atop the curve and overflows into the pool when the pump runs (pump pulls from pool main drain and skimmer, and returns to hot tub). When the pump runs, the hot tub fills and overflows nicely. The pressure is good, the water fall flow is nice, and there are no air bubbles coming from the jets. Once the pump stops, the hot tub slowly drains down a few feet. However, it never drains below the ports/inlets on the side of the tub. Because of that fact I am pretty confident the hot tub drain is not leaking (I have it closed) and the water is going back into the pipes through the ports / inlets.
I think perhaps I introduced an air pocket or siphon when I cut open the pipes and replaced the caps. The pump obviously was not running when I made the repair. Has anyone come across this or can recommend a course of action?