Mysterious little lock showing up on android apps that won't let me close them

Appears in the top right of the app screen when I try to close it. Various apps show it and I have to do a factory reset at this point to get rid of it. Nothing else fixes it, not shutting down or rebooting. If I uninstall and restart, then reinstall the offending app, it comes back - so it's something on board doing it.

That's obviously not right. I want it off.

How do I get rid of it forever?