To reproduce:
1. Open attached project
2. Create new Tile Pallet
3. Expand "Tileset.png" file and observe as there are 63 sprites inside it
4. Drag and drop the main .png file "Tileset.png" into newly created tile pallet
5. Observe as only 46 tilemaps are created

Expected result: all 63 tilemaps are created
Actual result: a lower number of tilemaps than actual sprites are created

Reproduced on:
2017.4.0f1 2018.2.20f1 2018.3.0f2 2019.1.0a12

Notes:
WORKAROUND
Selecting all sprites but not the main file creates the correct number of tilemaps. Only reproduces if the main .png file is dragged.