That happens to me on occasion when I'll be on the phone through the truck and I adjust the volume with the knob or steering wheel controls.
I think the red bar you're talking about is the minimum volume it'll play the call through the truck.
The problem is when it appears you lose your volume and control over it. You turn it up or down and the blue bar moves, but your volume stays ****-near muted.
I haven't figured out what causes it to appear, as it's appeared under a multitude of different circumstances(running Car Play or phone unplugged and listening to radio).
The problem is the only way I've found to kill it is turning off the fricken vehicle. So if you're on the freeway, say for a while(I live in SoCal), you're freaking stuck with no radio or anything that runs through your infotainment unit, unless/until you shut off your fuggin motor, which as I said before, isn't the safest thing to do while you're humming down the freeway. I did it last week when there wasn't anyone around me(was concerned my steering would lock), but it's not feasible to expect to be able to perform this maneuver on a regular basis so you gotta pull over and stop.
I asked the dude at my dealer who's in charge of this stuff('Product Specialist'), and I might add that he's the most knowledgeable person I've ever spoken to on the subject of vehicle based electronic hardware and software, but he could only speculate what it was, outside of saying that the infotainment unit is 'dumb', therefore he thinks it might be caused by my phone.
Unfortunately it does it regardless of whether of my phone is on or off