Hey Sam, I have a latest pre ASPH version of the 90 Summicron.
I found, that the lens indeed back focusses by about 1.5 − 2 cm in the close focus range, when shot wide open.
It works spot on though, when focussing from about 2m on.
I suppose, this is in the design of this particular lens and one has to compensate for example by slightly correcting focus or leaning back a bit after focussing, when shooting this close up.
Should your lens show mis-focussing at normal working distances for this wonderful portrait lens, you should check first your camera's RF and if in doubt, have the lens adjusted by a technician.