Why is the alignment marker for the 1.8 kb fragment not visible when using the 25 bp/1.8 kb DNA Size Marker together with the 15 bp/400 bp marker on the QIAxcel System and QIAxcel Advanced?

When using this specific marker combination with the QIAxcel System or QIAxcel Advanced, i.e. for STR analysis, the 1.8 kb fragment lies outside of the range of detectable fragments and does not show up on the electropherogram and/or the gel view image.