[2D] [Tilemap] Isometric Tilemap Cell Size resets to default when a new Tilemap is added to the Grid

Steps to Repro:
1. create a new Isometric Tilemap
2. change the Cell Size from the default X=1, Y=0.5 values
3. add another Tilemap to the existing Isometric Grid
4. check the Cell Size values for the Grid

The Cell Size values revert to default X=1, Y=0.5

Note: This behaviour is specific to Isometric Tilemaps; Rectangle and Hexagonal Tilemaps keep their modified Cell Size values