Grapher [PENDING ISSUE SINCE 2015]: Can't open saved file!

After you create a 2D-graph containing inequalities, save it, and close it, it becomes corrupted and cannot be opened again.

Steps to reproduce (example):

Step 1. Open the "Grapher" app, and select "Choose" (you will create a new default 2D graph).

Step 2. Delete the "y=" in the equation input box, replace it with "y>0", then press Enter. Now you can see the area of "y>0" plotted in blue.

Step 3. Close the window, and save the file somewhere on your computer.

Step 4. Right click the newly created file, choose "Open".

Step 5. "The document "Untitled.gcx" could not be opened."

I'm personally using macOS Big Sur 11.0.1, but I have seen a number of community posts here (since ca. 2015) regarding similar issues, and I couldn't find a solution for that.