The bug
The Executor returned by net.minecraft.Util.backgroundExecutor() (Mojang name) creates threads with the name "Server-Worker-X". This is misleading because the executor is also used for other tasks, including rendering and results in logged messages like:
[13:55:32] [Server-Worker-15/WARN]: Bytes mismatch 304768 0Which has been logged by com.mojang.blaze3d.vertex.BufferBuilder.clear(), a class used for rendering.
Classes have always had strange categorisations, but I think they are changing this sometime soon for modders etc.