Thank you for your post!
user14047 offered a great first step of trying iOS SDK 15.0.3. If issues persist just report back here.
I'm also going to move this thread from the FileMaker Community Feedback Space to the Discussions Space where others might find your post easier and offer further advice.
Thanks for the pointers from user14047 & TSPigeon (and for moving it to where I should have been posted).
I have now done a build with iOS SDK 15.0.3
The problem persists however.
Further information that I did not give earlier: The memory is released when the app moves off the layout which contains the pie chart. The data that the pie chart uses is unchanging while the app is on the offending layout.