No key layers in MapLike[{}]; No key structures in MapLike[{}]; Not a registry ops

Java version :

openjdk version "1.8.0_262"
OpenJDK Runtime Environment (build 1.8.0_262-b10)
OpenJDK 64-Bit Server VM (build 25.262-b10, mixed mode)

I recently updated a minecraft server from 1.15.2 to 1.16.2. I get this error message every time the server starts (before "preparing level" starts):

[main/ERROR]: No key layers in MapLike[{}]; No key structures in MapLike[{}]; Not a registry ops

The world is a superflat/void, so I don't see any issue with the terrain generator. I doesn't seem to affect anything so far.