And from #6431 to #6434 I also received "none" even though for the blocks after and before I did receive rewards. I was asleep though so I do not know if it was getting shares or not, and haven't looked further.
Your case is worse than mine. I only missed the reward on #6431 (135266).
True, #6420 was invalid. I'm just surprised that I got a reward on that one. There was a group of three that have rolled off the Block History. I don't understand why the reward for these invalid blocks weren't "none" but I'm not complaining.