I'm joining late, so may be missing something, but will offer that of many cases in TFP threads where spas drain slowly to pool level, small check valve leak is the only cause I've seen. (My pool, too.)
My bet is on the one with the "bypass" label. It could also be the 3-way just beneath it, but that doesn't seem likely since it's new. A thing to try is carefully disassemble the check valve, clean the seat (where the flapper rests) with a paper towel, add a layer of silicone grease to the flapper seal after also cleaning it carefully, and reassemble.
What happens now and then to mine is a bit of a seed or a leaf gets stuck on the seat or flapper seal. Water can scoot around it. This is what you're trying to fix.
Note on your pics: There are two pipes labeled "spa suction." I'm pretty sure the one on the left should say "spa return." Otherwise the plumbing doesn't make sense.
The pool level appearing to stay steady is a mystery. This will change as the ratio of pool's surface area to spa's. So if the spa drains 5 inches and has 1/20'th the surface area of the pool, the pool level will go up 5/20, a quarter inch. Not much.