You register your game rule statically in your mod's main class:

which is clinited in parallel with other mod classes. In that, you call GameRule#register, which adds an entry to a non thread safe map, creating the potential for a concurrent modification exception if two mods happen to register game rules at the same time. This issue would be hard to debug, and not be easily reproducible.

This is easily solved by wrapping the field in a memoized supplier, like so:

then calling get() on the main thread, like in an enqueued Runnable in the common setup event. You could also make the field non final and set the value in such a Runnable.