It has to do with the Raster Tools Mouse Settings..I had this problem as well and found a solution elsewhere. 

https://blogs.rand.com/support/2015/04/autocad-hold-down-shift-to-deselect-does-not-work.html

This corrected my issue of not being able to deselect when trying to stretch. Under raster design options, user preferences, mouse settings-uncheck the "Shift+Left click image select". You can find this under the Raster Tools tab in the ribbon and then click the diagonal down arrow in the lower right corner of the "Insert & Write" tool and that will open the AutoCAD Raster Design Options menu.