I have been dealing with this for years trying to stay ahead of any damage to the floor.
The first problem is that the back side of the tub tends to allow water to flow out on the the floor when taking showers. We have used the sealant and guards.
The second I feel is more important. The shower can be a little hazardous for footing. The tub is bowed or raised more than it should be for drainage creating even more of a slip hazard than the average shower with these sloping sides.
I have been able to sneak a look under the tub from the crawl space but not a full view of everything since the opening is limited. There does not appear to be any skids and definitely no mortar bed causing the rise. I am guessing that the tub was forced into the rough-in and and the tub bottom was not set or checked prior to securing it in place and finishing the walls and trim.
I was thinking of removing the baseboards and loosening all the edges to see if the side wall will move any and ad weight to the tub to see if the slope changes.
Any guidance on fixing without ripping the whole thing out would be greatly appreciated!