Step to reproduce:

1. Open the attached project (tilemap-gone.zip)
2. Open 'TilemapTest' scene
3. Focus on the 'Grid' game object so it's in the center of scene view
4. Notice the Lava sprites are painted on top of the Snow sprites
5. Open 'Tile Palette' window, choose 'Rocks' tile palette and select the brightest lava sprite to be painted (second row from the bottom, right-most sprite) and choose the 'Brush' tool
6. In the scene, move the cursor with selected Lava brush above the painted snow tiles and notice that they change sorting as you move the cursor (see attached gifs)

Reproduced with: 2018.3.0a1, 2018.3.0a11

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Resolved as (By Design)

For sorting, users should try to atlas the Sprites used into a Texture
or
Opt for Individual mode in the TilemapRenderer.