I experienced the same phenomena in the following two tests and today the cause of this came to me: I start my runs at 1.3A. Near the end the amp draw is 0.9A. And this is more or less the change-over point from Single to Double pulse (that is Single pulse @ +/- 0.9A, Double pulse at +1.5A). And exactly that happened: At- or below 0.9A, it changes over to the extreme point of the Double pulse:
-High amp draw -> Voltage shoots up
-Double pulse area in extreme point -> low RPM
Mystery solved

Best regards,
Rodolphe
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