I'm not sure If this is the same bug but I have similar occurence:
I have one runepage called Runepage. I always edit that one to my liking, I always have that one selected.
When in ARAM, for example, I click the edit button to get into interface where I edit the Runepage. The dropdown in champ select claims I have Runepage selected, when I get into the interface, I am changing the Runepage, however the dropdown menu in the edit interface claims I have CREATE NEW RUNEPAGE chosen.
When I do my changes and save, it saves it into Runepage, it makes me play with the Runepage.
However should I try to choose the Runepage in the edit interface (even though it should've claimed I am editing Runepage in the first place without me having to change it), then it does ask me whether I want to 'save the changes' I made to the CREATE NEW RUNEPAGE.
I've learnt to just ignore it, I always have Runepage chosen, when I choose the edit option, I do not interact with the dropdown of the edit interface, I do my edits, I save them, they do not create new Runepage (even though it claims CREATE NEW RUNEPAGE is chosen), they save into my Runepage, everything is okay. It's just little bit confusing and annoying.
I do not, however, experience what OP says, it never opts into different one. Just If I made changes to alleged CREATE NEW RUNEPAGE, didn't save it (which would save it into Runepage instead of creating new one) and tried to switch to Runepage in the edit interface, then it would choose Runepage and set it back to what it looked like before the changes.
It may sound confusing, so I'll note that during this whole time, champ select interface still claims Runepage is chosen, it's just the edit interface which defaults to CREATE NEW RUNEPAGE.