Found a working workaround on MacOS Sonoma 14.5 (23F79). Saving / loading seems fine.
cd Downloads/PixelComposer\ 1.16.0.3m/PixelComposer.app # or drag the app icon into the terminal ./Contents/MacOS/PixelComposer
Note that you have to `cd` into the directory then run the program. Running the full path to the program outside the .app folder causes it to fail to find some resource files.