Toilet tank fills after Shutoff valve is off and FluidMaster fill valve also is off
Water is not magically flowing up-hill from the bowl to the tank...

It is almost certain that the shut-off valve is not... shutting off.

Consider the design.  My experience is that the type of valve pictured uses a rubber washer as the sealing surface.

In most applications, a tap (sink, tub, garden hose) carries flowing water rarely.  The rubber seal spends most of its life sealing off static water, whose oxygen is quickly depleted.

In a shut-off valve, the reverse is true.  Fresh, oxygenated water is constantly flowing over the washer.  As the surface of the washer deteriorates, the ability to seal against the seat vanishes.

But the shut-off valve isn't used, so the problem remains invisible.

Until the day it's needed, as in your case.

There's no guarantee that the next shut-off upstream won't fail the same way.  I've had to call the City to shut off my water service at the water main, so I could replace the main shut-off, the hot water tank shut-off, and the kitchen hot-water supply shut-off... 

And then change the faucet cartridge...