If a file contains cover art with unsupported ID3 cover art types (as used in ID3, ASF and Vorbis image tags) then Picard will save those images back with type set to "other". That's because the type is mapped to CAA cover art types, and there is no one-to-one mapping for this and the mapping in the other direction is not equivalent. So most ID3 types map to CAA other.

Picard should preserve the original ID3 type and write it back.