Today I was setting up another testing environment for our project. What I wanted to achieve to have the character (green circle) move only on the grid lines. I used the tilemap object for the grid and I split each block into 4 pieces. Then I assigned each piece a different collision box leaving some free space around the grid lines for the character to move. When I add the 4 pieces once (forming 1 block) it works just as it should (top left).
The problems starts when I add more than one block next to each other (top right). Seemingly random quarters of the block "lose" it's collision polygon. The image still can be seen, so the tile is there, but I have no idea why does that happen. The more block I place the problem comes out even more (bottom part of the image). The collision polygons do not overlap, and I also set the Tile width; Tile height properties right.
Edit: If this turns out to be a bug, then I'll file a bug report.
Edit 2: I double checked and the problem seems to come from the fact that the tiles gets deleted, like I place a new one over them. The tile size is 50x29px.