I wuold check that your source footage is really 50fps, not your composition. Select the video clip file in your project panel and go to File > Interpret Footage > Main. Tell us what is the Frame Rate of your video.
I assume that you are using Dynamic link based on the name of your composition so you must check also that you are working on a premiere timeline that matches the frame rate of your file.
Go to the Project panel and select the footage. Look at the frame rate in the info that shows up to the right of the preview thumbnail at the top of the Project panel. If it does not match the comp frame rate you have two choices. Use the File menu or right-click and change interpolation for the footage, which will change the playback speed of the footage, or, if you want to maintain the current playback speed, pre-compose the footage layer in the Timeline moving all attributes to the new composition, then open the pre-composed layer in the Layer panel and use Rotobrush on the nested comp.
50 fps is a nonstandard frame rate and you should not be using that for a comp unless you have a very special need to do so.