That is possible. Chris could have been testing for A12 after a write and not a read. If only writes are affected, 3F and FE would still fail.
I believe we now know exactly why some games do not work on the FB2. Chris (cd-w) on the Harmony team figured out that the FB2 never sets A12 to zero on the cart port. This seems to have at least two effects: any game which relies on zero-page hotspots (Tigervision 3F for example) will never switch banks on the FB2. Second, any reads or writes to TIA locations will be appear on the cart as Superchip writes ($F0xx) and will write spurious values to Superchip RAM. The lack of A12 changes will likely also affect Supercharger, FA, E7, CommaVid, and other RAM games.
Strange, my '3E3F' bank switching scheme seems to work fine on an FB2 and uses zero page hot spots. Are writes only affected?
I tried it out again yesterday, and it did not work. I could have sworn that it did, perhaps I am remembering wrong.