Just to update this, it is definitely a bug.
I was watching Cloud Atlas and I stopped it. When I restarted it and clicked on "Movies/TV", Cloud Atlas was selected and there was both a resume and a play button. I used the back arrow to go and check another movie I watching before. I clicked on the (i) for that other movie and both resume and play were present.
Then I used the back arrow to go back and clicked the (i) on Cloud Atlas and the resume was gone!
Cloud Atlas is a blu ray and that seems the difference between the ones that keep the resume and the ones that loose it if I move away.
But resume definitely works with these blu rays because if I don't do anything after I start the program and I click the resume button on the blu ray then the program does resume correctly.
The problem is just the program forgets the resume spot for blu rays if you navigate away from the info window.