That's very odd if both batteries & alternator were replaced and it's still doing that. Was the reason for replacing all 3 because of this issue or after.

 

It did not start doing that until recently. Batteries and alternator were replaced 8-10 months before this issue happened. It got worse today and took it to the shop. They’re going to test and may replace the alternator again.

I’m wondering if the sensor on the negative terminal is bad.

 

Parts, OEM or not, are not what they used to be 20 or so years ago. An OEM part can be be new or remanufactured/rebuilt (reman for short). New or rem parts can be good or they can be utter crap.

In the case of an alternator, one bad diode can cut the voltage down at idle. The diode can be good when the part is in the box, and can test as good when the engine is off. But it can break down under load. Or the diode can just plain fail after a few months or a few miles, say hundreds to thousands.

Here are some short examples, for other parts, on YouTube. But the problems are widely applicable.
Bad machining. Bad fuel pump. Brake pads bad right out of the box.

 

The way I see it, OEM is the best way to go, BUT.... What is your source for the part? If you are starting at Amazon or eBay then you are exponentially increasing the odds of a counterfeit part. Stick with OEM sources. Mopar.com and your dealer. Mopar.com now offers aftermarket parts under the bProAuto line that supposedly are OEM quality for a lower price if cost is the main consideration. Play with the selected dealer on Mopar.com and you may even get a better price.

Rainman Ray did a video a few months back about a customer that brought in what they thought was a genuine Motorcraft part purchased from Amazon that was counterfeit. Where you buy it matters!

 

About 5 years ago I bought on Ebay what was supposed to be a set of 4 NGK Spark plugs for another vehicle, after 500 miles started to run like crap, after many at home diagnostics I finally pulled them out and they basically crumbled in my hand.. Now I only pay full price walking into Auto part stores in my town.