Mover ensure triggered when engine is running in the background

When the engine is running in the background, the following ensure may be hit in NetworkPredictionService_Interpolate.inl:

npEnsure(ToFrame == INDEX_NONE || LocalFrame - ToFrame < Frames.Buffer.Capacity());

This has been reported as happening in both standalone and editor instances, and the amount of time before the ensure is triggered may vary.

In a level with characters using the Mover component (such as L_CharacterMovementBasics from the Mover Examples Content plugin), enable network emulation using the "Bad" profile, and make sure "Use Less CPU when in Background" is true.
Start a networked PIE session with a server and one or more client instances.
Tab out of the editor, pushing it to the background.
After some time, the ensure should be hit.