So there was a change in the Command UI in the recent snapshot (19w46a) and now it will show the exception text above the command if the label (first literal argument) is wrong.

Looks like this change broke something! now if a argument doesn't have any suggestions, it shows that exception text and if you start typing you can see the argument name again, it wasn't like this in the older snapshots.

In the first screenshot you can see the error it shows, and the second screenshot shows how it disappears after you start typing, and this not good because it can confuse some users

One of the vanilla commands you can see this effect on is the tellraw command (because the third argument is a greedy string and doesn't have any suggestions)

also please ignore the fact that the server i was in is modified, its a client side issue