I would guess (only a guess, worth less than you paid for it) that someone had to dig out the main drain to replace/fix it and just left the "spoils" there when done. If that is correct, then there is no harm in putting it back (if you are convinced the drain is working correctly, not leaking, etc.) Loose dirt is not going to be doing anything structural. And, of course, if the mounds of loose dirt don't touch the joists they also aren't doing anything.

The proper path forward is to get a structural engineer out for an inspection and consult.

And definitely keep an eye on that drain - are you sure it isn't leaking or easily obstructed so that the previous owner wanted it easy to get to?