The translation of "Chain" as a name of a block is taken from the translation of "Chain" from type of command block. In Esperanto it's "Ĉeno" for the block and "Ĉene" for command blocks. But Crowdin marks the name of the block gray for some reason. Also, seems like other langs have the same problem. There are Russian comments about that.
The bug isn't there in 1.16.5, so it appeared in a snapshot.