I have the same build! How strange. I'm on a Nexus 6 (stock 5.1.1), UK based but SIM-Free (not carrier branded). See my other comment for a screenshot of what I see.

It's frustrating as I have to click the menu button, then Delete, then OK. I used to just hit Trash icon and that was it. For one-off deletions it's only a little slower - for working through a days worth of shots (or worse, high speed drive shots transferred from my camera).