It's a bug (hopefully a known issue, but I personally haven't submitted a bug report for it). If you look at the bottom of the item, you can still see room for the name, but it is missing (whereas an item bought from an NPC doesn't have this extra space at the bottom). Also, if the item(s) in question show up on your lodestone profile, the name is present, so thanks to that, you can rest assured that the name data is indeed still there, the problem merely lies in the display of the names. Hopefully they fix it soon.