How precise is your adapter?
That is not going to cause focus shift.
An adapter that is not on-axis will cause a shift
if you focus on one side of the frame and then
reframe the subject over to the other side. This
would happen even if you focus at the working
aperture. If focus changes in that manner, that
is not actually "focus shift", which refers to the
focus shifting due to stopping down.
But if you focus and don't reframe, you can still
get a shift from the lens itself, same shift you'd
get even with a perfect adapter. If the adapter
has an incorrect flange distance, but is on-axis,
that will NOT cause a focus shift. It will cause
an inaccurate reading on the focus scale, but
focusing at wide open will remain correct when
stopped down unless the lens has a focus shift.
Focus shift is usually due to poor placement of
the iris along the optical axis.