Tile preview image not updated when manually adding URLs

[stinos]

For a while I had the impression newtabtools would, seemingly randomly, not create thumbnails for tiles newly added. However I noticed thumbnails being created in profiledir/thumbnails for pretty much all sites visited, also the ones for which the tiles don't have a thumbnail. So I looked for why these tiles don't get a thumbnail and found a way to reproduce this:

• open new tab tools options, use arrows to navigate to empty tile
• type a URL manually (not selecting from dropdown), e.g. https://github.com, then click Add and close options window
• click tile to visit newly added site, thumbnail gets created on disk
• close tab, open new tab tools: thumbnail isn't shown, and tile title is just the URL

However, in the second step instead of typing the address manually, one of the dropdown addresses is selected, the title is already filled in correctly at that point and moreover in the last step the tile gets a thumbnail. In other words, to continue example above: if I add github.com as a tile by selecting it from the dropdown I get a preview image on the tile, but if I type the address manually I don't get it.