Hello,
Dispatched operations of a planned order are deallocated when you convert a planned order into a production order (process order). If the dates of the planned order and of the new production order (process order) are not the same. Might be some change is being made on the quantities/capacity during the conversion, what makes the order to be de-allocated.
In the capacity planning table, have you set strategy 'Insert operation'?
Please refer this discussion,
Dispatched Planned Orders become deallocated upon conversion
Best Regards,
R.Brahmankar