I am running the latest Kubuntu (25.04) and have installed the latest Processing via Snap.
I like to open .txt files with Kate, Kubuntu's onboard text editor, so the default is set as such:

Obviously, I want to use Processing to open .pde files, so I indicate that as my preference on this menu:

However, once I do that, Processing is now the default app for .txt files:

Please let me know how to fix this!