To work around this issue follow the steps below:
file my_input : TEXT open READ_MODE is "file_io.txt";
1. Click on "Add Sources".
2. Select "Add or Create Simulation Sources".
3. Select all of the .txt files which are required to be read or used in your project and click OK.
4. Ensure "Copy Sources into Project" and "Include all design sources for simulation" are selected.
5. Click on finish.
6. These files will be added to the project.
The files will now open properly even with relative paths.