Zero forecast in DP does not set forecast to Zero in ECC completly

Dear Experts,

We are in SCM 7.0 EHP 3, and facing below problem.

for Example: if we set forecast to Zero completely in DP and then transfer to ECC.

forecast in ECC does not get wiped out completely(which is expected) instead first 8 weeks it will  zero out the forecast and from 9th week on wards  old forecast remains intact.

could you please advise if any one faced this issue and solution ASAP.

thanks,

Sunil