Export BandMath image

Is there anyway to use an image produced in BandMath? I can’t seem to export it using subset and it doesn’t export when I save the file to Geotiff. And I can’t draw a polgon over the features the BandMath image has highlighted.

Maybe it can help if you convert the created band.

Right click on it and select “Convert Band”.
Initially it is just a virtual band which still references the source bands. Converting it, resolves the equation and the data is fixed.
You can deselect the creation of virtual bands when using the Band Maths.