Manger field values is overwriting after incremental crawl in SP2013?

Step 1: we ran full crawl successfully. Step 2: we updated manually manager values for 10 users. Step 3: removed mapping for manager in Manage User Properties. Step 4: Again we ran the incremental crawl.

After incremental crawl completed,manager field value is showing empty for 10 users in the SharePoint User Profile.

we removed mapping for Manager even though why it was showing empty value once incremental crawl is completed.

Can any one give response on this ASAP.

Thanks in advance.