Lightroom exports keyword's synonyms to Flickr only at first publishing. New synonyms are no longer published at following publishing. Of course, the photo is actually published again, the synonym is marked as exported (check box).
LR version 6.3 with Adobe plugin. The problem is reported on the Flickr forum and on the Adobe forum (Adobe Lightroom 6.3 Flickr (Default) Plugin : Synonym keywords not being published to Flickr.). When tags are added to a photo by an other person than the author, I understand that to remove tags is an issue, therefore full synchronization is not to be desired. The first time keywords all are exported. Then, new (different than existing) tags could be added safely.