Spotlight search support on macOS

My question is about Xmind support for indexing and searching maps from Spotlight search on macOS. If I try to search for map content, the search gives no results. Issuing

mdimport -n -d1 map.xmind

in Terminal reveals

Imported 'map.xmind' of type 'org.xmind.openformat.xmind' with plugIn /Applications/XMind.app/Contents/Library/Spotlight/XMindSpotlightImporter.mdimporter.

So a Spotlight mdmporter plugin is available. But running

mdimport -n -d3 map.xmind

reveals among other attributes

kMDItemTextContent = "Warning\n\n\U8b66\U544a\nAttention\nWarnung\n\Uacbd\Uace0\nThis file can not be opened normally, please do not modify and save, otherwise the contents will be permanently lost ...";

which is not the actual textual content of the map.

Could that indicate a bug in Xmind's mdimporter plugin?