Nuke roto shapes gets deleted with UNDO command

[prashanth-v-srinivasan]

Hi, 
Does anyone face problems with 'undo' command with 'roto' shapes. 

While doing Roto, when i used 'undo' command few 'roto shapes' got deleted and few got duplicated randomly. This is wired and it has been happening again and again at random days. 

Nuke 7v8 is being used. 

Any help or workaroud is highly appreciated. 

thanks