Steps to reproduce:

1. Open attached project (tilemap-gone.zip)
2. Open 'TilemapTest' scene
3. Focus on the 'Grid' game object so it's in the middle of the scene view
4. Drag the scene view to the right slowly
5. Notice that the entire tilemap disappears in the scene view, as if it's being culled too early (see attached gif)

Notes:
You can still paint on the tilemap, tiles just aren't visible in scene view (can be seen in gif as well).
Doesn’t happen on Rectangular and Hexagonal tilemaps, only 'Isometric' & 'Isometric Z as Y' are affected.
Affected by the ‘Cell Swizzle’ parameter on the ‘Grid’ component, if changed to something like ZXY - disappears when moving the scene view down, not right.

Reproduced: 2018.3.0a1, 2018.3.0a11