This issue has been fixed with the latest version of the visuals, which is currently being processed by the MS appsource team, this process will take about two weeks. However these visuals are available to download from your ZoomCharts account > Power BI BETA visuals - https://zoomcharts.com/en/account/dashboard/pbi-beta/
So if you download and import the BETA visuals into your reports, the issue you mentioned will be fixed.
The reason was a combination of few things, starting with the app itself not being updated for the iPad Pro release, which was done last week https://powerbi.microsoft.com/en-us/blog/power-bi-service-and-mobile-February-2019-feature-summary/ ending with some capabilities that needed to be adjusted within the visuals themselves to stop them from unwanted resizing, which caused the visual to flee from screen.
Let me know if you have any further questions.