I was giving some thought to your problem this evening and it hit me.
both egc and ungrounded conductor were landed on the ground bar in at the tub
This is a no-no to start with. The grounded (neutral) will have a lug of it’s own and the equipment grounding conductor will hit a lug that is attached to the metal of the control panel.
but in the disconnect I found the the egc was not attached to anything and the ungrounded conductor was capped off (looks to me to be no ground at all!!)
You are correct, there is no equipment grounding path at all installed this way. These people have been very lucky for the past year.
The tub had a sticker that stated it only required three wires.
This means that the grounded (neutral) conductor is not needed at the tub and can be capped in the tub panel.
I placed the egc in the disconnect gound bar and then once I placed the ungrounded conductor in the appropriate lug on the GFCI breaker it tripped immediately.
Here I ask, was the grounded (neutral) conductor still terminated in the control panel of the tub? If it was and the equipment grounding conductor was connect at both ends this gave a parallel path for the current to return and thus the GFCI did it’s job by tripping.
I suggest that the grounded (neutral) (white) conductor in the cable be left off at both ends. The grounded (neutral) (white) conductor of the GFCI installed and the equipment grounding conductor installed at the appropriate places and see if this cures the problem.