Tested on 1.13.3, but it seems this is still valid in later version (although, I am not sure).
It seems that the culprit is in app/assets/javascripts/host_edit.js -
When you select a compute profile, it will call computeResourceSelected, which will return the right image selected, but then, down the flow, it will call update_capabilities() which will call update_provisioning_image() which will empty the image selection and repopulate it with a call to foreman_url('/compute_resources/'+compute_id+'/images'), thus forgetting the selected image from the profile.