Well I suppose that's not really fair - you can add them but you can only see the first one in the approval list, and on the detail/results pages. To replicate: Add a tag, such as [U+1F60D] to a biblio in the OPAC try to add another emoji, such as [U+1F3B6] In my 18.05, this won't work and the second tag is never inserted into tags_approval, though it is inserted into tags_all. It seems to be mysteriously approved as under the first emoji added, as well. I checked the DB encoding, it seemed correct. The database I was testing with was an upgraded one, 3.20 -> 18.05.02
Further to this: if you try to add one emoji, then another different single emoji, it won't do it. But if you tag with two, it will work. But then if you try another set of (different) two emoji, it won't save it, and so on. It's like it doesn't see the emoji as unique characters and the strings only become detectably unique once more characters are added. You *can* add the same single emoji that initially saved to a different record, and increment the count for records with that tag. That works fine.
Also appears in master.
Retested, still an issue. Adding Tomas.
Please QA 21846 and this will be history. *** This bug has been marked as a duplicate of bug 21846 ***