I had the same issue. Working with external files in a Xojo Project, edit some code of it and save the project file results into the fact, the code is no longer editable, until you close and re-open the Project again in Xojo.
I have seen this on a few occasions - always when trying to edit an external item. Of course I thought it odd but I just rebooted Xojo and all went away. I’ll pay closer attention next time.
I don’t think they’re editing it outside Xojo, but when they edit an external item inside Xojo, then Run/Build, the external item is no longer editable until reloaded.
Like others, I’ve seen this once in a while and it was never reproducible. I’ve even had external items, where I could edit the code, but the changes were not included when running the project, nor saved when I saved the project.