Changes to Object Mask reoccurs?

Hi all,

I am pretty new to premier pro and are using the beta version as I need to mask out a person using the object mask tool. 

It does a pretty decent job, but the mask need some refining. For example it creates holes inside the mask, these are easy to fix by going frame by frame and using the lasso and the plus button. When I fix one part of the mask then track it backwards and forward then hit save, the changes are there. 

If I then move a bit further into the video to fix another part of the mask that the object masking tool did not catch, It performs the change, but the previous change I did has come back. So the new changes at for example 6 seconds in is there and the change I did at 2 seconds has reappeared. 

Am I doing something wrong that makes premier forget previous mask changes?

Method

Scrolling through clip to find mask imperfection. Fix these with lasso tool frame by frame. Track backward and forward. Save. Move playhead to new section of mask imperfections and perform the same task. 

Thank you.