"Mushy" transition from OFF to ACC

Yesterday I was making some measurements of current and voltage from the battery for the Prius at different states of operation. Looking for values close to C/10 and C/20 for use in testing battery capacity. (My previous method used a current load closer to C/2, and that was a mistake because of Peukert's law.) Anyway, with a helper in the driver's seat the car was twice transitioned from OFF to ACC while voltage was measured at the posts on the battery. There were other measurements too in each group, but the battery was allowed to relax back to (nearly) the first resting voltage over a period of about 7 minutes between groups of measurements, and OFF to ACC was the first measurement in each group.

Observation 1: 12.68V to 12.35V (2.40A)
Observation 2: 12.64V to 12.42V (2.40A)

Slightly odd that the final voltage went up .07V between tests.

The end point voltages don't really show what happened though. When the Power button was pushed one time, to enter ACC, in both runs the voltage immediately dropped some, but then it fell for the next 10 to 15 seconds (I wasn't expecting it and so didn't time it with a watch, that interval is an estimate) at a rate of about 0.01V/s until it stabilized at the lower voltages shown. None of the other tests did that. The transition from ACC to IG-ON was over in a second or two, and things like changing the fan speeds just jumped from one voltage to the other more or less instantly. Turning on the headlights with the car OFF resulted in an instantaneous transition, and that pulled 2.93A, which is close to 2.40A. Suggests that it is the car and not the battery which is "mushy".

I thought about all of those videos of people sitting in their cars checking the auxiliary battery on the MFD, and I don't think they would see this . The MFD only shows 3 digits, so the .01V/s drop would only show up as a single change in voltage. Secondly it takes a while to get to the final voltage display on the MFD, which will have let most of the slow change complete before the voltage is read. I have a little voltmeter plugged into the power port under the dash, and I just looked at it while doing OFF to ACC and it showed 12.60, then 12.50, then 12.40 over about the same time interval as described above. (The device can measure 4 digits, but here it acted like it was only doing 3 and tacking a zero on the end. Oh, I remember, that voltage is always 3 digits, it uses the 4th for power or current when a USB device is plugged into it.)

Anyway, can anybody confirm this mushy OFF to ACC voltage transition? If so, what do you think might be causing it?

The ACC to IG-ON transition current change was complex and time varying, it briefly jumped over 20A then fell back to 9.06A, but the voltage change moved from one value to the next in less than a second.