QCAD start,
Application Preferences:
Units: inch
Dim: Fractions

Changed the Drawing Preferences to:
Units: mm
Dim: Decimal

Design a Dim and it will be created with the mm/Deci settings - as expected,

Select now the Dim - activate the ISO Tool - the try to determine the reference point fails / it's way of the clicked point and now the produced ISO Projection will use the attributes for DIM's from the "Application Preferences" setting - hmmmmmmm, not as expected :-(