Spa mode back draining during operation

:wave: Welcome to TFP!!!

Please define "operation".

If you are truly in spa mode, then 100% suction should be from spa and 100% return to the spa. A check valve only prevents draining when the pump is off.

If the spa is draining in spa mode with the pump on, then 100% of the return water is not to the spa ... check the position of the return 3-way valve.
 
A couple of things to check.

1.) Return 3-way diverter seal is ripped. You can check this by pulling the diverter apart. Rare but I've seen it happen.
2.) Chlorinator or Polaris is plumbed in a way to allow return to the pool in spa mode. I've also seen this more times than I'd like to admit. The suction to the Polaris pump is plumbed in-between the filter and heater and because it returns to the pool, when in spa mode it drains the spa by returning the spa water to the pool.

Some pictures of your set-up would be extremely helpful.
 
You probably have a suction side actuator that has failed, or quit working. This usually happens when you use the spa. Both actuators rotate correctly, you use the spa with no issues, then when you are done, you put the system back into pool mode and only the return side rotates back to pool. You don't know that the other actuator has not rotated until the next morning or maybe later, if you don't look at the pool often.

This is easy enough to check. Place the system into spa mode and check to see if both actuators are rotating. Just be sure that you can get out to the equipment to see this after pushing the spa button. Most systems will also let you put it into spa mode right at the equipment, however you must place the system into service mode first, with the exception of the Easytouch system.
 
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