When adding tags

When I try to add tags to other people's works in the gallery, the number of tags I can add is limited.
I can use tags I've favorited or popular tags like "girl," but when I try to search and add other tags, they become unselectable and I can't add them.
This worked fine when the tag addition feature was first introduced, but it stopped working after a few days.
I think this is probably a bug, so I'm reporting it.
I'm sorry if this doesn't make sense. If that's the case, please let me know.