The following bug exists as long as I can remember and still exists in Fairphone OS 1.5.1:
I realized that I couldn't change my cellular data cycle (see left screenshot). So I tested around a bit and found out that after activating cellular data, I had to do another action (e.g. switch to the seconds SIM card) to get the option "Change cycle..." (see right screenshot). That dropdown disappears again when I disable cellular data and do an action again. Can you confirm this?
Since this is a pretty obvious bug, I thought somebody had posted it in the forum already, but I didn't find any such post. Give me a heads-up if I didn't search well enough.