Received a replacement 5268AC today, and tried using the bridge mode on this new modem. Unfortunately it has the same problem -- the bridge mode appears to be non-functional.
I swapped in the old 4111N modem and everything works.
At this point, I have a high level of confidence that the bridge mode on the 5268AC does not work.
I've only tested with this one firmware version though, so maybe it's a bug in the firmware?