Dear Community,
To give you a short and promising update:
I discussed about this issue with Apple Developer Support and then finally raised a bug report.
With 17.4 Beta, the problem now seems to be resolved. For ink-annotations including UIBezierPaths, obviously some distance between annotation boundary and path boundary is now required to avoid a change in size after saving the PDF. But then it works fine under 17.4. Also for stamp-, circle- and square-annotations, everything seems to be fine again in iOS 17.4 without any need for code modifications.