I have upgraded to QGIS 3.4.4 and using the georeferencer and there appears to be shift when zooming in and out of a georeferenced image (Transformation type is Helmert, Resampling Method is Linear and CRS is 27700) which I haven't had on 2.18. Also getting colour issues when zooming out as well. Not sure if anyone has had the issue or knows If I'm doing anything wrong.
I had a similar issue with raster data when I displayed layers with different coordinate systems within one QGIS-project (using spontaneous projection/ on the fly projection). So maybe it might help to disable the on-the-fly-projection an to use only layers with a same coordinate system that is the same as those of the georeferenced image.