"Force download" unable to be changed once selected

It is possible to be locked out from changing the "force download" and "window" settings for resources. Steps to reproduce:

1. Admin => Modules => Activities => Resource => Change Window from default "Same Window" to "New Window"
2. Edit any course => Add a resource => Link to a file or website => Change Window from "New Window" to "Same Window"
3. The Force download checkbox that was previously locked is now available. Set it to True. Save.
4. Edit the resource just created. Both "Force download" and "Window" fields are locked. If you wanted to change these settings, you would need to delete the link and start again.

The bug is only apparent when the overall admin default is set to New Window.