No bug. through story you will learn something that allows you to progress. It is sort of a way to force you to come back at a higher level because what you will fight within.
I think you're talking about the door to finish level 13 and the OP is talking about the very top right corner of the map where there is a treasure room. I am quite sure this is the result of the rope vision you get after killing Maerwald. If you take the knowledge, this is the knowledge that you get. I made that choice and was able to open the door. I am, however, unable to open the door to progress and I guess you saved me some searching.