Could cheese "halt" the tenderness of cooking lentils?
I have always been told that high acid-yielding foods can cause dried legumes to stay hard. I can't say that I have ever added Gouda (although it sounds delectable) but I have had dried beans stay hard when adding tomatoes too early in the cooking process. Wondering about this, I looked up Gouda and was quite surprised to find that it is a high acid-yielding food. 

Could be other reasons such as legumes that are old or a hard water situation but I suspect you have already considered and eliminated those as you are very thorough. :)