How to reproduce it:
1. Open the user's attached project
2. Load 'BaseLevel' scene from 'Assets' > 'Scenes' folder
3. Enter Play mode
4. Observe that 'Player' Prefab collides with the Tilemap and doesn't fall down
5. Exit Play mode
6. Build and Run this project

Expected result: 'Player' Prefab collides with the Tilemap and doesn't fall down through the floor
Actual result: 'Player' Prefab does not collide with the Tilemap and falls down through the floor

Reproducible with: 3.1.6 (2020.1.0b6, 2020.1.8f1), 4.0.2 (2020.2.0b10, 2021.1.0a4)
Could not test with: 2019.4, 2020.1.0b5 and lower (due to errors)

Note:
- Reproduces on Mac and Windows Standalone builds
- Works if you apply Composite Collider 2D to the 'Tilemap' GameObject
- 'Player' Prefab collides with 'GameObject' GameObject, which has a Box Collider 2D
- Tilemap's sprite is imported with PSD Importer, which does not have Generate Physics Shape value