It is unclear, because the rules as written (RAW) do not address this.
In this situation, a Floating tensioner disc it floats on a surface, and the surface suddenly becomes a hole. Unfortunately, the rules do not provide guidance on what exactly happens next, as the disc is not expected to be above a hole more than 10 feet deep.
The mechanics of the spell suggests that it was not intended for this scenario, as it normally "floats 3 feet above the ground" and "can move across uneven terrain, up or down stairs, slopes, and the like, but cannot cross a change of elevation 10 feet or more. "
The spell description does not specify the speed of how fast the disc moves, either horizontally or vertically. The disc itself is a "horizontal force plane", which may or may not mean that the plane is fixed in place.
So there are several possible outcomes, depending on the interpretation:
The disk is not stationary and would therefore fall off like any other object affected by gravity. It falls until it floats 3 feet on a surface, or exceeds the maximum distance of the caster.
The disc is affected by gravity, but "cannot cross an elevation change of 10 feet or more." If the hole is more than 10 feet deep, the disc remains in place. Otherwise, the disc falls off, as described above.
The disc is a magical force and therefore gravity does not apply, so it remains floating in place.
Finally, the DM would need to decide what happens.