Yes, this has been talked about before. It's not a bug. The F4U-1 had the P&W R2800-8 double wasp engine which had a decrease in fuel consumption during WEP due to a water injection de-riching mechanism. That train of though was carried over to the F4U-4 model. The F4U-4 pilots manual I have doesn't show a decrease in fuel consumption at all operating conditions, but it's close to the F4U-1's engine operation.
The F4U-4 pilots manual I have expects the pilots to figure out their planes fuel consumption. Numbers are printed, but a caveat is made that all numbers need to be field verified. I don't think any pilot is going to do that during combat conditions.
Regards,
Malta