Still no contradiction. It is fair to assume some things that we don't see on screen still took place. We know that for Obi-Wan, Yoda was 'The Jedi Master that once instructed me.'...and Yoda did. We know this because Obi-Wan said it and Yoda does nothing that contradicts this. We just never saw his instruction on screen.
If you really want to get technical about it, the words used by Yoda and Obi-Wan would suggest that Yoda was not the one that trained Obi-Wan. Follow me on this:
Obi-Wan says 'instruction' when speaking of Yoda's involvement in his education. Yet, when Yoda is working with Luke, he almost exclusively uses 'train' or 'training' which suggests a deeper involvement than 'instructed'.
Also Yoda going from living in the big city to living as a hermit was explained on screen. He had to go into hiding as his life was in extreme danger, so he had to find a place of safety. Also, his ability to see events clearly using The Force was clouded. He needed to find clarity through peaceful meditation and solitude...to re-center himself with The Force. He picked a place that would provide both.
There is no contradiction... technically or otherwise.