In minecraft 1.16.1 (Minecraft_server.jar), I have found an issue revolving around display names in items. 
If an item's JSON information exceeds a certain length in characters (Not sure which it is), the idem's display name is not applied.

The command used: https://hastebin.com/vohowepeya.makefile
The expected result: 
The actual result: 

There isn't a maximum length for a line of text's JSON information on the tablist, bossbar, action bar, scoreboard and Message Of The Day so I assume that this is unintentional.