My mentor told me to stop rounding my spines after 12 years of doing it

I was at the bindery in Portland last Tuesday showing a new hire how I round spines and my old mentor stopped me mid cut. He said I had been doing it backwards for a decade, that I was rounding before the backing was fully set. I argued with him for 20 minutes. Then I tested it on 6 practice blocks and every single one opened flatter and held its shape better his way. Twelve years of muscle memory down the drain in one afternoon. The weird part is nobody ever corrected me before, not at the two shops I worked at or any of the classes I took. Now I have to unlearn the whole motion and start over like a beginner. Has anyone else had a mentor catch something basic you had been doing wrong for a really long time?