So stick with me on this....

My FB is supposed to always block.  In my playbooks he is always blocking.

When my RB1 got fatigued (which happened much more in the game today than normal), GD put in my fullback, and NOT the backup RB1.  Instead, the backup RB1 went to fullback??!!

I would think that the backup RB1 would fill in for the starter.  What am I missing here?

You could have your FB listed as second on one of the RB depth charts, and that particular formation was pulling from that particular depth chart.