If you use the clone command on a tile entity (for example a hopper), you see an output like this in the console:

[00:35:26] [Server thread/ERROR]: Block at -8,246, 36, 1,554 is Block{minecraft:barrier} but has net.minecraft.server.v1_16_R3.TileEntityHopper@3faed881. Bukkit will attempt to fix this, but there may be additional damage that we cannot recover.

Steps to reproduce:
1. Place a tile entity (Hopper, dropper, chest, etc)
2. Use the /clone command to clone it to somewhere

I found with testing that doing a simple clone command such as
/clone -8246 36 1553 -8246 36 1553 -8246 36 1554
does not produce an error the first time, but if you do it again, then you get the error.

Also, you always get the error if you add the args "replace move" to the command:
/clone -8246 36 1553 -8246 36 1553 -8246 36 1554 replace move

I found this earlier issue which is marked as Resolved but it seems to have returned.
https://hub.spigotmc.org/jira/browse/SPIGOT-5472

I also found this related commit from late 2019:
https://hub.spigotmc.org/stash/projects/SPIGOT/repos/craftbukkit/commits/75fac431cfc807ccf05629ce4796db958ee112e6

Thank you, have a nice day.