Array of actors forgetting its contents

Weird problem. An array of actors is forgetting its contents every now and again.

I have a few light switches in a level. Each light switch is a blueprint that contains an array of light blueprint actors that are turned on or off when activating the switch. Every few days my level stops working,. and when I check I see the array on each light switch is now empty. It loses its contents and I have to set it all up again. This has happened 4 times in the past week.

Any ideas? The actors are not being spawned, or respawned on load game or anything, just simple blueprint actors, left untouched. Until they break. It also only seems to happen when the array is of a blueprint actor, not an array of a simple variable type.