Issue:
In Reaper 7.56, saving modifications to a loaded preset in Pianoteq VST does not update the original file [Preset Location: C:\Users\[Username]\AppData\Roaming\Modartt\Pianoteq\Presets\My Presets ]. Changes are saved to an unknown location (Project/FX Chain), causing presets to revert to their unmodified state when opened in Standalone mode.

Solution:
Reinstalling Pianoteq 9.1 resolved the issue (likely caused by a corrupted installation).