Reading other posts, folks mentioned that the printer wants to see 210mm of travel from the top stops when moving down.  In the software, if the PINDA sees the bed before the steppers think they've travelled 210mm, then the Z calibration fails.  I tested this by removing my extruder nozzle and fan nozzle so they wouldn't hit the print bed, so I could move the PINDA upward.  Sure enough, it went from fail to working.  Unfortunately when I reinstall the extruder nozzle it will hit the print bed before the PINDA can read the bed.  Thus the conclusion is that my printer's travel is just under 210mm.  I double checked my extruder nozzle was fully threaded in, that the head is assembled properly, and that the Y axis all-thread rods and seated all the way down in the frame.  No dice, the stack up of parts is such that I don't have 210mm of Z movement.  I suspect others that have done the retrofits on their Mk2.5 printers are on the edge like I am.

I wish PRUSA would simply take the 210mm setting / check out of the software.

When firmware is 2.5s the XYZ Calibration fails and I notice that the first calibration circle is off by at least 5mm on the Y axis.   3.12.0.5672
When firmware is 2.5 the XYZ Calibration succeeds and I notice that the first calibration circle is accurate and not off.  3.11.0.4955

The firmware for the 2.5s has incorrect parameters in it?