Try it with a two engine bomber, it's always been that way.
Midway
With the two engine bombers, yeah, I'm aware. I'd think something wrong otherwise.
The thing is, when I first started playing (back in Tour 113), any time one of my 4 engined drones lost an engine, it would start to slowly fall behind. Of course, when it lost 2, it would quickly fall behind. Now (at least the last few times it has happened to me), if one of them loses an engine, it will keep up, no problems. If it loses a 2nd engine though, that's when it will fall behind quickly. This is me at full power mind you. If memory serves me, this started to happen to me after they made a change to the bombers, a change about how they gunned. Hell, I can't remember WHAT they changed though, but it was around that time.