Thank you for providing your sample spreadsheet and code snippet for our review.
We are able to observe the problem on loosing conditional formatting rules when rows are deleted from the range on which condition formatting rules have been defined. Please note, we used the latest version of Aspose.Cells for .NET 7.7.1.1 for our testing. A ticket (CELLSNET-42318) has been logged in our bug tracking system to probe further into this matter. Please spare us little time to properly analyze the problem cause, and to provide a fix at earliest (if applicable). In the meanwhile, we will keep you posted with updates in this regard.
Please accept our apologies for your inconvenience.