If an Edge Terrain has both corner and edge labels, using it produces incorrect results that differ from how it works as a clean Edge or Corner terrain, acting more like a Mixed Terrain but not quite:

When using a Corner Terrain that also has edge labels, using the random terrain Fill gives results akin to Mixed Terrains, using tiles in the Tileset that don't even have corner labels, but not connecting them the way Edge terrains would connect:

These behaviours are unintuitive, and both seem to stem from the Terrains using labels that are irrelevant to the current type, instead of only using the Corner or Edge labels.

Although one solution is to simply automatically erase irrelevant labels when changing types, I think it would be better to keep them and just not use them, so that users can more easily experiment with the different types.

Related issue: #3204, deals with displaying unused labels. I guess it's appropriate that unused mixed labels are currently displayed as a mess, since they behave like a mess :]