Hi Mehdi,
Have you got any solution for this problem?
We are currently facing this same issue and have referred note:1973926 .
We are currently at MDM 7.1 SP09 pack so not sure if this defect is corrected in our current version.
We have all the attribute type as Text and the values are not getting set a blank in DataManager.
If you can help us it would be much appreciated.
Thanks
SCN User