How to fill 3/4″ gap between tub and tile floor

I'm afraid your shower plumbing is leaking again. Not as badly as before, but leaking none the less. From the looks of things, water getting past the tiles from showering wouldn't amount to much, and would have certainly diminished by now due to lack of use. Continued flow indicates a water source that is not dependent on showering activity.

The reason you are not seeing water on the kitchen side is because the leak is fairly small and the water found somewhere to go that has enough flow capacity such that the water does not overflow across the wall baseplate or subfloor into the kitchen side.

Bear in mind this is just my best guess. Water leaks can be difficult to trace to the source. No matter what, something is not right and needs to be corrected. Even if the source is something else, you need to get into the wall to investigate. Presumably this is easiest from the kitchen side. Would this be how the plumbing was accessed to repair the previous leak?

No, that's not sufficient.

I would epoxy a flange to the tub. And then for good measure use some roofing tape (which is flexible) to backstop the flange. And maybe for really good measure install a pan under the tub, or a the very least some building paper leading to a visible spot below (so you quickly detect any leak).

Remember that most building materials can survive getting wet periodically. No building material can survive staying wet. Trapped water means mold, rust and worse.