Sealing 2-ft square hole in cement beneath tub
Well, that's a code violation, at least under IPC.

1002.8 A recess provided for connection of the underground trap, such as one serving a bathtub in slab-type construction, shall have sides
and a bottom of corrosion-resistant, insect- and verminproof
construction.

Additionally, it sounds like the tub (with a 2 foot square hole under it) may not be &quot;bedded&quot; - which is OK if it's cast iron, and otherwise dubious - plastic and thin steel tubs should have material filling between the tub and the floor (plaster is traditional, some folks use sprayfoam these days) to prevent it from flexing (and eventually cracking) when loaded with water and a person's weight.