If I put a rectangle with 80% opacity over part of an image in InDesign it makes the image brighter.

As stated in the title, placing a black rectangle set at 80% opacity over a section of an image brightens the entire image.

The black translucent rectangle is being used to make text on the image more legible without completely blocking out the image in that area. 

The issue is still there if I move the translucent shape onto another layer, the only way I've found to make the image return to normal is to delete the translucent shape.

Nothing is grouped, the image opacity is still set to 100, and I'm working in Europe General Purpose 3 for the colour settings, and it stays when the document is exported to a PDF.

If any other details are helpful to diagnose this issue, please let me know. 

Is this a known bug or is it affecting the properties of the page somehow?