@Nachti,
We were able to observe the issue but we need to look into it more. We have logged the issue in our database for investigation and for a fix. Once, we will have some news for you, we will update you in this topic.
This issue has been logged as
CELLSNET-46963 – Chart labels lose formatting after saving Excel file
The issues you have found earlier (filed as CELLSNET-46963) have been fixed in Aspose.Cells for .NET v19.11. This message was posted using Bugs notification tool by Amjad_Sahi