Which desktop version did you use to create the report? Based on my test in Power BI desktop latest version 2.46.4732.721 64-bit (May 2017), the Percentage format of the measure will not lose once reopen the .pbix.
Please update the desktop as ours. Also you can download the attached .pbix file then open in your environment to see if the Percentage lost.
Best Regards, Qiuyun Yu
Community Support Team _ Qiuyun Yu If this post helps, then please consider Accept it as the solution to help the other members find it more quickly.