For some reason the Tile Painter GUI doesn't appear when I select it from the toolbar or use the keybind. I can still select an area and it applies the tile settings it last had. It seems this issue only applies to specific saves of a world I'm playing, even after removing all other mods and loading the save anyways, and all earlier saves work fine, so I don't think its an issue with another mod. Though I'm pretty sure I used the GUI at some point after the earliest save with the issue. Maybe it has to do with something that changed in Factorio 2.1.13 or 2.1.14?

I managed to fix the issue by disabling Tile Painter, saving the game, and then re-enabling Tile Painter. I wonder if the game somehow recorded the position of the GUI somewhere off-screen, and disabling the mod reset it?