THE BUG:

1. Load a single image.

2. Select part of your image with any of the select tools other than Free Select, e.g. use Rectangle Select.

3. Switch to the Free Select Tool.  Hover anywhere over your image, press and hold the CTRL or SHIFT key: notice the plus/minus does not appear in your mouse cursor to indicate you can add/subtract from the existing selection.

THE WORKAROUND

1. Load any second image, e.g. duplicate the first image to a new tab.

2. Switch back to the first image and perform same selection steps above to confirm the CTRL/SHIFT keys do not alter the mouse cursor indicator.

3. Switch to the second image then immediately back to the first image.  Now press the CTRL or SHIFT keys while in Free Select Tool (with existing selection) and notice that the mouse cursor correctly changes to show a plus/minus cursor indication you may modify the existing selection.