I know it occurs in German between southern and northern accents where the r at the end of words is silenced in the north. I'm sure it occurs in other languages

I know it occurs in German between southern and northern accents where the r at the end of words is silenced in the north. I'm sure it occurs in other languages too. I think you're confusing vowel shifts with rhotacism. A non-rhotic speaker will not pronounce the r in either "her" or "father". It's quite possible the vowel shift at the end of father happened in response to the disappearance of the rhotic r, but it is a separate effect.