To reproduce:
1. Open my attached project "test", SampleScene
2. Open ProBuilde and it's UV Editor
3. In Game View, select Cube GameObject
4. Switch to Face Selection mode
5. Select single face of theCube GameObject
6. In UV Editor, change Fill Mode of the selected faces from already set Tile to Fit
7. Select all the faces of the object and change the Fill Mode back to Tile

Expected: selected faces are set to Tile Fill mode
Actual: selected faces cannot be change to Tile Mode

Reproduced in: 2018.1.2f1, 2018.2.0b6

Edit: if step 7 works as intended, try Undoing one step and changing the mode to Fill again. There's always be a mode which you cannot change into