Structure block data length limited to 12

I wasn't able to reproduce. By "data field," do you mean the the data structure block?

The data mode in the structure block, yes. This does not happen on single player, you must be on a server. I've attached a video of this happening.

This error occurs for me whilst trying to use a Structure Block on my 1.16.2 server.

Any random string can be entered into the "Custom Data Tag Name" or "Structure Name" boxes, up to and including 12 characters without issue (and the structure spawns as intended). However, when 13 or more characters are entered, and "Done" is selected, the game throws this error (simultaneously kicking me from the server), and the block is not updated from the previous successfully set value.

This is greatly frustrating, as it means the vast majority of structures can't be loaded in!

I have uploaded the error message received.

this seems like a simply typo, somebody at mojang missed their 8 key or something 😃

fix is here, https://github.com/PaperMC/Paper/pull/5700/files