This is obviously a regression caused by changes in Thunderbird Core. It is caused by code injected via
messenger.WindowListener.registerWindow("chrome://messenger/content/FilterEditor.xhtml",
"chrome/content/scripts/qFi-filterEditor.js");
is not loaded in time so the appropriate functions don't get monkey-patched before the editor is filled with actions / search conditions.
The code works as expected when manually adding a quickFilters specific search term or action, but the filter editor is incapable / fails to add these correctly from a saved filter, making once generated filters impossible to edit.