Your original polygon might have been too large (ie, spanned more than 162 degrees) - see here. One way to test is to see if it can be filled (by enabling Fill polygons in its green layer). If it can be filled (as a solid colour) but not filled as a raster then it’s probably the 162 degree limitation.
Maybe it works now because it’s smaller than before?
It’s true than polylines can also be used. Although users are encouraged to use polygons (by stating Reconstructed polygons) since they are guaranteed to be a closed loop, whereas polylines can look like the following (where a single coastline boundary consists of multiple polyline segments)…
We probably should not allow polylines, but there are cases where users have polygons in polyline format and it would be a hassle for them to change it.