From the UDN thread:
Turns out this is caused by the way this stat was implemented for the FBehaviorTreeInstance struct: 1) You call IncMemoryStats on construct, before the struct is populated with data. 2) You call DecMemoryStats on destruction, but at this time they are including the size of all ActiveAuxNodes/ParallelTasks/InstanceMemory in the calculation.
Results: In most cases you will end up subtracting more than you added, resulting in an overall negative stat value being accumulated & when it rolls over you end up with
Map: any map with multiple npcs
PIE: open console and activate ai stats: "stats AIBehaviorTree"
The memory counters for instance memory should no longer be very large numbers
counters for template and instances should be stable according to the amount of npcs spawned
There's no existing public thread on this issue, so head over to AnswerHub just mention UE-94207 in the post.