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Thankyou so much for fully explaining Rosc. Your idea of leaving the resistor in place is certainly the better way to do it. I misread the datasheet, thinking that having 10K can also give the same effect as shorting to Gnd (without knowing the stock resistance).
The 10K stock resistance should be 12uS, according to the formula tOFF = Rosc / 825. It gets set to 30uS when grounded. But I guess it's not the same as automatic mixed decay. The faster tOFF time might not be equivalent to auto mixed decay.
For the record, I don't know with absolute certainty if R49 is Rosc. I counted anti-clockwise from pin 1 of the IC to find it. If it was actually clockwise from the top of the IC, then I actually grounded the 'Step' pin. That would explain why the motor wouldn't go on open circuit
The first 2 attatched photos show the left and bottom edges of the chip, but that would be irrelevant if it's the right edge that has pin 13