3d helmets I've dropped in my safe-house to display have consistently vanished from existence, examples being Hard Hats, Police Motorcycle Helmets, and Fire Department Helmets. My guess it's probably getting wiped like 3d items knocked off of zombies heads / face.
EDIT: Learned from the Reddit that this is not a bug but instead part of Item Despawning settings that are on Apocalypse by default, my suggestion instead is to exclude player placed items from this filter.