I just caused an ‘invalid’ tag to turn up, which I figured I’ll explain because it might help someone in case they see it.
I often use screenshots (jpgs or pngs) in my videos, which I typically rename before importing them into Shotcut.
This time I renamed a file AFTER I had already imported it into Shotcut (no other details were changed).
This ‘broke’ the file as far as Shotcut was concerned, so if you see the behavior (whether or not you’re working with image files) consider looking at your order of operations.
This is because in a Shotcut project file, it writes XML code using the address of that file. If you change the name, or move it, you will get “invalid” 100% each and every time.
When you save/restart the project, it gives you a screen to locate the proper file(s) that are missing.