S9 Plus Volume Flucuation

[Android Central Question]

I purchased a Samsung S9 Plus late last year (Carrier T-Mobile). The phone worked great until the March 2019 OS update. After this update the volume fluctuates low to where I had it set (or high), every 6 six seconds when the phone is plugged into a car infotainment center and the phone battery was charged less than 100%. When the battery charged to 100% and the lightning bolt inside the battery display in the upper right corner goes off, the volume will then only fluctuate once every 8 to 10 minutes. This volume fluctuation happens in two different vehicles with two different infotainment centers. The infotainment center in my personal vehicle is a Kenwood Model Number DNX574S. The other vehicle is a company pickup truck. It is a 2016 Chevrolet Silverado with their stock infotainment center. I only use the Samsung OEM cables in both vehicles. I tried swapping the cables to no avail. I also ordered 2 brand new OEM Samsung cables and tried them, again same result. Since March I have been complaining to T-Mobile concerning this issue. T-Mobile has told me repeatedly that they have never heard of this issue and I’m the only one with this complaint. The volume of the phone does NOT fluctuate in any other audio conditions/connections, bluetooth, headphones, phone speakers or AUX cable. T-Mobile finally agreed to even exchange my S9 Plus with a band new one. The exchange took place today. And guess what happened? When I plugged the brand new S9 Plus into my Kenwood infotainment center and played a random video on YouTube, the volume fluctuated low to where I set the volume (high). I tried a couple of other apps like Radio.com and Amazon Music, same result. T-Mobile is telling me that they do not know what to do for me since they never heard of this issue. Any help that can be provided in solving this volume issue will be greatly appreciated.