I ride road, recumbent, and MTB's on a regular basis. I have power meters on the road and recumbents.

After adaptation (months), my FTP (measured at 30min) consistently differs between the road and two styles of recumbents ("mid-racer" vs. "high-racer"). I find my maximum sustainable power comes when I ride sitting very upright on the tops of my road bars -- it definitely isn't the most aerodynamic position, but I suspect I can breathe easier in the more open position and get better cooling (either on the trainer or outdoors) that way.

If I haven't been on the high-racer recumbent in several months, my first few rides on it will feel very constricted. My power numbers won't be far off their normal, but the effort feels twice as hard. After a week of regularly riding it, everything will be back to normal.

I wouldn't be surprised if the extra width of the MTB bars is letting you breathe more freely than typical road bars/positioning. My MTB's are setup that if I put my hands on the bars close to the stem (inside the shifters/brake levers), it feels very roadbike'ish. Some MTB bars can be exceptionally wide, and I've found I need to cut mine down (contrast that with wanting the biggest width road bar), or at least move the grips in, to be all-day comfortable.