When the Ore Processor's output is full but it still has items to be processed in its input it will consume Essence without processing anything.
Small video for clarification: https://youtu.be/-RyXOOKYYS0

Minecraft: 1.12.2
Forge: 2542
Industrial Foregoing Version: 1.5.5-92
TeslaCoreLib Version: 1.0.11
Mods: list.txt