If the extension uses manifest v3 and a background script in the form of a service worker, then in Safari it is not possible to open the background script debugging window. If I expand the Developer menu in Safari, there is nothing under Web Extension Background Data (or disappear after click), which is an error. In other browsers (Edge, Chrome, Opera, Firefox) this works correctly.
If I switch the background script back to non-persistent script mode, everything works fine and from the Developer menu and the Web Extension Background Data submenu I am able to open the background script debugging window for the extension. Am I doing something wrong?
Our engineering teams need to investigate this issue, as resolution may involve changes to Apple's software. Please file a bug report, include a small Xcode project and some directions that can be used to reproduce the problem, and post the Feedback number here once you do. If you post the Feedback number here I'll check the status next time I do a sweep of forums posts where I've suggested bug reports.
Bug Reporting: How and Why? has tips on creating your bug report.