Just wanted to throw this out there....

Was running a similar issue last month. Battery held 12.6v overnight, but went flat during drive after about 20minutes. I checked voltages...
no load: 13.8v
load (lights + radio): 12.8v

I load tested battery, checked out...
Replaced VR....no change.

I ended up replacing the battery and the problem was fixed...hasn't returned since I guess the battery was functioning and holding a charge fine, but as soon as it had load on it, it couldn't handle it.

Since you have already replaced the alternator recently, I would say look at the battery. How old is it? Mine was 3 years old. Maybe buy a new one and if doesn't fix the issue return it.

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2016 Monsoon Gray Audi Allroad - 21k
2008 Black Mercedes E350 4Matic Sport - 131k
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2003 Gray Mercedes ML350 - 122k