If you set the suction side to draw from the spa/hot tub, the spa should drain down unless the suction is set to 100% from the spa, the return valve is set to 100% going back to the spa, and the pool cleaner line is 100% shut off. Have you tried that combination: 100% from spa, 100% to spa, pool cleaner 100% off? When set that way, the equipment pad should be connected only to the spa, and there shouldn't be any way for the spa to drain.
The "pool cleaner" is the pipe between the two hot tub pipes that goes to a very small secondary filter and then to a valve in the back near the filter. It may or may not actually be connected to a pool cleaner. My guess is that you have that valve in the back set to allow some flow to the pool cleaner, and that is the path that allows the spa to drain.
There should also be a check valve in the spa return line. I think it is that gray T/90 degree corner near the automated return valve, though I can't be sure from the picture. If that check valve is not working the spa will be able to drain when in pool mode with the pump off.