Yes I'm aware of that - but the glasses work by having a layer that delays light entering the front of the glasses that's polarised 45 degrees off vertical by a quarter of a phase (converting CCW light to h-polarised, and CW to v-polarised), before passing through a second layer that's a linear polarising filter. Hence, CCW light entering the front is blocked, and CW light entering the front leaves the back as plane polarised.

Consequence - flipping a filter front-to-back turns it into one that blocks plane polarised rather than circular.

(eta, fixed out-by-45 degrees error on entering light)