Note that I do use a paper server for testing but the same bug occurs on a spigot server.
This does require an IDE for java development, a server you can test on, and a new java project with spigot and craftbukkit as dependencies to use nms.
Once this is done compile the jar and put it your server's plugin folder reload/start the server then run /npc-create. It will create an npc with a steve or alex skin and put a name into tab. Then run /npc-remove and it should remove the npc from tab and not remove the npc itself.
If you have any problems with the code please contact me.