With Zenphoto 1.6 and PHP 8.1 I receive the following warning for image flipping in admin area:
Caution: This will remove any metadata from the file due to your graphics handler (GDlibrary) limitation. Use Imagick instead if supported.
I checked with PHPinfo that GD (2.1) and Imagick (3.7) are both enabled. Is it possible to enforce Imagick out of Zenphoto? As I do not use flipping of images it's only abit annoying because it consumes more space now.

This is not ZP 1.6 or PHP 8.1 specific, GD just cannot handle metadata. You can switch to Imagick if available on Options > Image as always.