Spa draining into pool when in use

We recently moved into a new home with a pool/spa combo. When the spa is in use, it appears to drain into the pool, causing the spa water level to fall below the jets. I have read some about the potential for valves not closing all the way causing water to be sucked back into the pool, but in this case the valves are Pentair intellivalves that seem to be fully operable (based on the indicators on the top of the valves). Any ideas as to the source of the issue and how to fix? Thanks in advance!