Weighing in with my experience of using the strips. Bought them 11 years ago when making my first big wedding cake for my daughter. For that cake and ever since, have only used them on cakes 9” and larger. Even skip using them on the 9” many times, but always use them on a 12”. I follow that whether baking a butter cake or sponge-type.
Never had a problem. Until the luxury lemon cake I did for August 22nd wedding. I had exactly the result you describe. This time, I used magic strips on the 9” to be extra certain of a good result. Cake sides beautifully done, centre like batter by the time whole cake “should” be done. If I inserted a tester, centre sank like mud. Especially with the 6” and 9”. If I didn’t use a tester and just baked the suckers until the centres were clearly done, the sides were overdone. In the end, discovered that the culprit was the butter. It was one of those newish extra-rich butters, at 82.9% almost 3% higher in fat than my regular butter. Weakened the structure of the cake. Also affected the quality of the mousseline, which was looser than usual and hard to work with.
I apologize if you’ve already read about my experience in the other thread, and this is repetitious. But when I read your description, tofusalem, couldn’t help wondering if the butter is the culprit in your case, too. You’ve investigated all the other likely suspects (oven temp, etc.) Oh, one small method point to watch is to be sure you’re not over-filling the pans. Scale the batter into your pans by weight, using Rose’s recommendations on p 490 TCB.
Good luck! Keep us posted. Carol