I would suspect that the tub drain was improperly installed.
The outlet port for a tub drain is purposefully recessed down about 1/4" below the surface of the tub floor. When the tub drain basket is properly installed the top surface of the drain cover should be nearly flush with the tub floor.
To properly install a tub basket a small layer of plumbers putty or silicone caulking is put on the underside of the tub basket flange.
The tub basket is then inserted through the tub port, through a rubber gasket on the underside of the tub and into the threaded receiver in the tub waste shoe.
As the tub basket is screwed down into place the excess plumbers putty is squeezed out leaving the tub basket flange in tight contact with the top surface of the tub floor. The treading action then pulls the waste shoe up toward the underside of the tub compressing the gasket to form a tight seal on the underside.
Many DIY'ers and sadly some wannabe plumbers erroneously place the rubber gasket on the upper side of the tub directly under the drain basket, which then leaves the tub basket elevated by the thickness of the gasket, the 1/8" of an inch you mention.
If this is your problem you will see the edges of the black rubber gasket under the tub drain fitting.
If this is the case, it can be easily corrected from the upper side of the tub. Begin by removing the center screw and tub drain basket cover.
Using a tub basket wrench, unscrew the basket and pull it out.
You can then carefully insert the rubber gasket through the tub drain hole so that it is between the underside of the tub and upper side of the tub drain shoe underneath.
Roll out a rope of plumbers putty about the diameter of a wooden lead pencil and apply it to the underside of the tub basket flange, or you may apply a bead of Tub & Tile silicone caulking.
Carefully slide the tub basket down through the tub hole, through the rubber gasket and into the Tub drain shoe receiver.
Again using a tub basket wrench, screw the tub basket into the shoe until it is tight.
Wipe off the excess plumbers putty or caulking and replace the cover.
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