The most common cause for this is a frame rate mismatch. The layers' frame rates don't match the comp's frame rate.
Thank you for the response I wish that were the case... happening in 30fps comp, with 30fps footage and 30fps precomps.
Not a huge deal, just annoying, and was wondering if it was something with the way After Effects is coded.
I realize I can also just time-remap the layers and reverse keyframes, but would be nice if time-reversing a layer just lined up.
is it 30fps footage or 29.97 footage, which is far more likely?
It's 30fps footage. A 3D rendered image sequence... imported/interpreted at 30fps, rendered at 30fps. In a 30fps comp. Not 29.97 anywhere in this project, Dave