For shits sake lets simplify this.
1. Test constant hot to neutral. Constant hot is the black wire attached to both switches. One lead on the black and one on the white. Do you get 120V? If so step 2
a. If you do not get 120v your problem is before the switch.
2. Turn on the switch and test switch leg to neutral. Do you get 120V? Switch leg is the other wire on the switch.
a. If you do not get 120v throw switch in the trash and get a new one.
b. If you do get 120v the problem is either the wire underground or the light fixture.
Most pools have a deck box somewhere that the cable from the wet niche fixture terminates in. If you turn the switch on you should get proper voltage. If the spa light works fine I suspect your problem is after the GFCI.
Farts are funny
I'll look for a box in the bushes behind the light tomorrow, hopefully my problem is in there.
"WouId it be possible that there is an issue at the gfi causing this? My understanding is that the white, green, black hot wires are coming from there."
Now I'm confused. if your two wire is bringing in the power than your 3 wire would then be your switch legs. Therefore the black spliced jumper is wrong. Unless I'm missing something, you need to verify which is your power supply.
But if the 2 wire is bringing the power in, why is the black from the 3 wire jumping across both switches?
We have already covered this part and that's not how it's wired. . Left switch is first in line, jumper is not on the switched power. The issue is the red wire heading out to the light on the top of the left switch which goes from 36v(off) to 85v(on).
Does les the fact that 85+36 = 121 point to the problem by any chance ?