Rotobrush Selecting Entire Frame

The first thing to do if you are having problems with Rotobrush is to delete the Rotobrush effect from the layer, pre-compose the layer, name it "For Roto" or something similar, making sure that you are moving all attributes to the new comp. Trimming the pre-comp might be a good idea if the layer is not as long as the comp. There is no need to open the "For Roto" comp but you do need to open the layer in the Footage Panel using the Layer menu or a right-click and apply Rotobrush there. If you still have problems you might want to open up the "For Roto" comp and add some color correction to the footage so the edges are a little better defined. To tell the truth, I do a fair amount of color correction and garbage masking on most of the footage I'm going to rotobrush because it just works better on most shots if you do.

The only thing that is 100% certain, if you try and rotobrush a layer that has the time stretched you will get a warning and I just discovered that the entire frame will be selected in the latest version of After Effects if you try and use Rotobrush. I don't know if that happened in previous versions because I always make sure that the frame rate matches the comp's frame rate with I'm creating doing roto work.

If the comp was created by selecting a clip in a Premiere Pro sequence that had a speed change the footage in the speed change will show up in After Effects. You can see it in the timeline by enabling the In, Out, Duration, and Stretch columns by clicking on the {} icon in the bottom right corner of the Timeline.

Just pre-composing a Stretched layer will solve the frame rate problem but the frames will all be reinterpreted and the likelihood that you will get the best matte possible goes way down. The bigger the time stretch the lower the quality of the Matte. Pre-composing will get you to a place where Rotobrush will work, but it will work better if you are processing the original frames. There is a fairly simple option to end up with a comp that has the true frame rate of the footage and the correct in and out points.

The first thing to do is to make sure that Synchronize Time of related events is turned on in your AE Preferences. Now follow these steps:

1. Duplicate the footage layer in the comp created by dynamic link
2. Select the top copy and pre-compose but this time Do Not move all attributes to the new composition but open it. This will give you a composition that does not have a time stretched layer, but it also does not have any in or out points
3. Return to the main comp and put the CTI on the first frame
4. Return to the Pre-comp and the CTI will have moved to the same frame that is in the main comp
5. Set a new in point using the keyboard shortcut Alt/Option +[. 
6. Repeat the procedure for the out point of the pre-comp (Alt/Option + ]). You now have a comp with trimmed footage at the correct frame rate for roto.
7. Complete your Rotobrush and either render the result or use the Pre-comp as a track matte for the original footage

That will get you there and solve the whole frame selected problem. The choice is yours. If Pre-composing and moving all attributes give you a good enough matte then that option has the fewest steps. If you want to get the best results then use the checklist I just posted. It's the most efficient way I can think of to deal with a frame rate mismatch.

Not The Problem, Never Was. STOP ASSUMING THINGS. You are constantly leading people down paths of false salutions based off of pure assumption and have been for years.