Active tile 'changes to nothing' and back sometimes

Normally, you select a tile in the tileset. Then, the tile shown in blue under the cursor is that tile, and clicking plots it on the map.
However, occasionally, no tile is shown instead. (To clarify -- I don't mean a blank space that is colored blue, I just mean that no tile at all is drawn under the cursor). While this is the case, clicking on the map appears to do nothing.
This applies for (at least) Stamp Brush tool and Bucket Fill tool, and appears to be unaffected by whether Random Mode is on or off.

Any clicks on the map occurring after the initial 'loss of tile' may reset the 'current tile' back to the tile that it should be, or may not; the reverse also applies.
In all cases, the change occurs after a click or drag.

Steps involved in triggering bug:

1. Create new map, 100x100 tiles with 12x12px tile size, Orthogonal, CSV, Right Down
2. Add new tileset:
   
3. ??? (no reliable recipe yet. All I can say is that all of the occurrences began with the above two steps)

System details:

• Arch Linux x86_64
• WM: AwesomeWM
• Xorg: 1.17.2
• Tiled: Git commit 9c2be76
• Tablet: Monoprice 12"; lsusb identifies it as "UC-Logic Technology Corp. Tablet PF1209". It uses evdev input driver.
• input-evdev: 2.9.1.6+ (git b370ccd)

Start by following the reported new-map and tileset steps, then investigate the tile preview and map-click behavior for Stamp Brush and Bucket Fill. The issue has no reliable reproduction recipe, named files, or tests; done means identifying and fixing the intermittent loss of the active tile and confirming clicks and drags continue to plot or fill normally.