It happens due to the packet having the player internal post effect non-thread-safe list, so netty reads at the same time as main thread executing `/posteffect` writes to it A potential fix being one like in https://github.com/PaperMC/Paper/pull/14335
As a note this is necessary in order to preserve the correct types in data compound tag of the entity or the correct custom data on items in equipment/inventory
masmc05
It happens due to the packet having the player internal post effect non-thread-safe list, so netty reads at the same time as main thread executing `/posteffect` writes to it
A potential fix being one like in https://github.com/PaperMC/Paper/pull/14335