This is an image of south-western Bulgaria (https://www.openstreetmap.org/#map=11/41.9776/23.4126).
Right in the middle there is a line, to the north there is a full layout with forrest cover, to the south there is nothing like it. How can this be fixed?
Looking at the data this was an import by dido3 with comment Рила: полигони в периметър K-35-071 от bgmountains, част 2
This was truncated at 42N, that explains the straight line with no rendering south of it, there is no data in osm south of 42N. It is not unusual that imports like this are done in discrete lat-lon sections.
You’d have to ask dido3 why he didn’t continue. The import is 3 years old.