It's not indexing in my case; if you take the stick out it'll re-index, but if the stick is left in it doesn't need to do that when you next turn the car on.
It does the same with radio stations too.
I'm 98% sure (without actual knowledge
) it's a glitch in the "remember settings per key" functionality; regardless of whether the glitch is trying to switch-over even though that function's supposed to be turned off, or the glitch is that the function is on & it's switching to the wrong memory/setting.