I think the happens in any single engine aircraft, but I was flying in a p-40E when I noticed it.
Steps to reproduce:
1) Get to altitude in your single engine aircraft and run at full power (no wep).
2) Hit x to enable the auto-level
3) Hit shift + 2
4) Watch your RPM and manifold guage while pulling back the throttle.
Your RPMs and Manifold will stay steady, but if you watch your VSI and Altimeter, you will see that you are descending. You have lost power, but the guages do not indicate it. I believe your airspeed also drops, but I didn't look specifically at that.
One of my squadies noticed this and showed me it last night. I don't think it is anything that can be exploited, but it is an interesting issue.
To recover from this, just hit CTRL + E
