I have the same issue. Its only when the mission is on the Daily Hard/Challenging/VIP list. The context menu that allows you to pick the difficulty doesn't show up. All other missions work fine.
I get it when the same mission is on 2 of those lists. I haven't checked to see if it happens when its only 1 of them.
Today I have Camp White Oak, it is both Daily Hard and Challenging. I cannot select the difficulty, but I can play it one normal when I go through it.
Not to long ago, I had one that was on the daily Challenging and VIP mission and it wouldn't let me play it, the following day I went back to it to check and it was normal.