The red valve and the check valve next to it are what is called a make-up line..
This allow water to go to the spa when in the pool mode. This is why your spa overflow into the pool
If you close the red valve (Good luck with that.. as ball valves last about 10 minutes) the spillover will stop. But.. if the drain down stops when the red valve is closed, it confirms that the check valve next to it is leaking.
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