Hi @toby,
That’s very strange and I can’t reproduce the issue on my MacBook Pro running 15.5.
Each release of Processing is code signed and notarized, so it shouldn’t be quarantined. And as you said, macOS would normally display an alert if that were the case.
Could you try Processing 4.3 and Processing 4.3.1 and let us know if the same issue occurs with those versions?
For accurate results, make sure to close all running instances of Processing before opening a different version.
Is there any additional info you could share about your setup? Anything out of the ordinary that might help us narrow it down?