I would guess that the tub may not be properly supported/bedded - thin steel and fiberglass tubs need a layer of support between the tub and the floor. The tub probably has noticeable flex when you step into it, and this flexing is breaking the enamel off. Traditionally plaster, or spray-foam (polyurethane) these days quite commonly.
If you have any DIY inclinations, the "tub-refitters" will not seem so inexpensive. They are only (possibly) inexpensive as compared to a full-on contractor remodel, if then. And the result is often not satisfactory (do some web searching.) There are some real horror stores out there. I have personal experience only insofar as getting figures in the $4000 range to "refit" a tub that was failing, and reading enough about problems to opt not to go there.
The new tub was $250, new plumbing was $80, new surround was $350 after I saw what the $170 version looked like when I got it home (much too thin and cheesy, returned and upgraded.) A wee bit less than $4000... It's messy and takes a while, but this is "one of the tubs" so at least you have another one you can use while ripping this one apart - which would be my suggestion.
In the meantime, you're fine to use this until it actually rusts through and starts leaking. I don't hold out much hope for the enamel repair, but it's worth a shot.