How to reproduce:
1. Open the user's attached "TilemapCollisionBug.zip" project
2. Open the "SampleScene" Scene (Assets folder)
3. Make sure the Scene view is visible
4. Expand the "Grid" GameObject in the Hierarchy window
5. Select the "Tilemap (2)" GameObject in the Hierarchy window
6. Observe the Scene view

Expected result: Correct collision geometry is applied to the Tilemap, no tiles are left without a collider and there are no holes in the collider
Actual result: Two colliders appear on the Tilemap, as the Tilemap gets wider the collider gets more inconsistent leaving the lower part of the Tilemap without a collider and
eventually leaving a hole in the center of the Tilemap

Reproducible with: 2019.4.5f1, 2020.1.0b16, 2020.2.0a18
Could not test with: 2018.4.25f1 (When downgraded, it can be seen that the geometry collision is applied correctly, but after unchecking "Used By Composite" and then checking it again, no geometry collision is applied at all)

Notes:
- The issue is not reproducible if the Composite Collider's 2D "Geometry Type" property is set to Outlines
- The issue is not reproducible if the Tilemap Collider's 2D "Used By Composite" property is unchecked