I have the same problem. In my case its the yeast. I wash my yeast and try ro get the most out of it. Pretty much after 3 months in the fridge without using it is about the life of my yeast before things go sour. I have been sticking to SO5 for now and no sour beers this far.[emoji2]
If i remember correctly when i brewed this, it wasnt all that great till about month 6 for it to start to shine though. Could be the yeast hasn't settled yet and that is what your picking up in the flavor.