Unfortunately, I cannot be sure whether this is an iOS issue, or and iSimple issue. However, I just tried the 3 steps above and they worked. Here's the deal with my issue...
GMC Sierra 2004. I have been running the same iSimple gateway since my original iPod (circa 2013 iPod). Then same iSimple connected to iPhone 4. Then same iSimple with lightning adapter since the iPhone 6's came out. Last week, for the first time, this volume icon display issue started...and I have not done anything different through all these years in terms of hooking up, powering on radio, etc. It completely varies as to what order I do them in (radio on first or not), but I have never ever disconnected the lightning cable from the iSimple cable...not once since I originally bought it.
I took iPhone to Apple store, they said they've only seen this issue when the phones get wet or damp. Under AppleCare they replaced the phone no hassles. This new phone has the exact same issue. Today I narrowed it down to this:
I can plug in the cable to phone (lightning already on iSimple cable). So long as I don't switch to the AUX 1 input of the radio (where the iPhone would normally play through), the Volume icon does not appear. The second I switch to the AUX 1 source, the volume icon appears. Here's the really strange part (other than the fact this JUST started up last week after all these years).
If I pull up Music, and am in the "For You" section, the volume icon will remain. But, when you put it in full screen for that track (where the album cover is large), the volume icon fades away...! Then it'll come right back when you leave that screen.
Sop, this issue is not resolved. But I am leaning towards this being an iOS issue. I just find it hard to believe that the same hardware would suddenly start acting up, when it doesn't have an effect on any other function.