Hope this spares others from wasting time and money on a new battery and alternator.
@fordraptorfun You saved me wasting time on a trip to the dealer at any rate. Appreciated.
Was getting p0563 for
high voltage. Watched the voltage meter and never actually saw it out of range. With the high warning the truck didn't do anything like you described, just pinged the message and lit up the battery warning lamp - saw this post so I checked the BMS wire - didnt see any corrosion but figured I'd tighten it. Turns out it was totally loose and the nut was backing itself off. Tightened it up, issue resolved.
Pretty sure it's on the original battery so presumably it's been like that from the factory. Recommend giving it a quick tighten next time anyones under the hood.
Edit: For posterity's sake - This came back two days later, worse than before. So it dawned on me the dealer did for sure disconnect the battery at one point last year - and after going back and spending more than 5 seconds looking - it appears they installed the small BMS wire
under the much larger (alternator?) terminal w/ a washer. Since the bigger terminal/washer was hitting the diameter of the BMS terminal crimp, the smaller BMS terminal was kind of floating around under it - and probably contributed to it backing off in the first place. Swapped, no more problem. So long story short, small wire on top, righty tighty.