using the roto brush on linked compositions from premiere pro

Dynamic link doesn't really work and causes so many different crash and bug issues that its almost impossible to troubleshoot.

You are best off cutting and exporting the 40 frames as a new file, putting that into After effects (or preferrably an editor with rotoscope that doesn't crash) and then once completed, exporting that and importing it back into into premiere as a new clip.

Rotobrush can be very messy to work with. When I'm editing in Premiere Pro and I need to use Rotobrush I create the comp using Dynamic Link, save the AEP file, then I go back to Premiere Pro and undo so the original footage is back in the timeline.

Back in AE, I do the roto on the frames that I need, then I create a new comp to do the composite and nest the roto comp in the new comp. If the Rotobrush area is longer than just a few frames I almost always render the footage to a 10 bit or better DI (digital Intermediate) with an Alpha Channel and replace the rotobrush layer. Here's my reasoning.

Rotobrush is system intensive and if you are not careful or forget to freeze it will propagate again and foul things up. Rotobrush layers add significantly to the AEP file size. Anything that could slow down the render or the preview in Premiere Pro can be problematic.

When I have completed the composite I create a new Bin in Premiere Pro called VFX, import my rendered composite, and drop it directly above the original footage. That is the least destructive, safest, and fastest rendering option.

If you only want to see the frames you are working on when you use Rotobrush just pre-compose the footage and run rotobrush on the pre-comp. That problem is solved.

That's how AE works when rotosciping/tracking etc. You can either zoom in on the region of intrest with the bar above the timeline

or pre-compose your footage and then open that precomp in the layer view (Right Click > Open > Open Layer) and roto it that way.

i can only use it on the source footage that was imported (which is very long).

rotobrush works in the layer panel. you are supposed to see the whole source but only the parts that are trimmed highlighted. this is supposed to work just fine and the rotobrush span is supposed to not pass to the untrimmed parts as long as you keep it trimmed in the layer panel. show some screenshots or a video capture so we can see your process.