After you enabled time remapping, you should change the duration of the layer such that it spans the entire duration of the composition. After that, any mask on this layer should also stay there over the entire duration of the comp.
I tired this and it does not work. It won't let me change the time. So my other question is do I need to time remap before I create the mask? I can insert the time remap just not change the time. If I add the layer and time remap first then create the mask after that might work? Could that be why or not? I will test this.
If none of the above work. Try leaving it all as is and pre-compose the layer with the mask and time remap this. That should work (You might want to use frame blending as well)
The point is that it does not work with a predefined mask before TIme Remap.. It will work if I time remap the layer first then create the mask afterwords.
I am testing this one more time to verify this and let you know.
Edited a few minutes later:
Doesn't work. Timeline sais 3f so I have to assume the MOV is 3 frames. Its at 00:00:00:03 and won't let me change it to anything else. Frame bending doesn't seem to do anything either frame mix or pixel motion.
Edited a day later:
Can anyone of who has replied try to simulate what I am trying to do with a 1-3 frame MOV file?