I have the same issue, since updating from bios 49 to 53, my USB keyboard is not recognized during boot/post and I cannot enter the bios unless I get out an old keyboard with a PS2 connection.
My keyboard is connected to the rear USB 2.0 ports, I did not try connecting it to a USB 3.0 port yet. (Thanks for the tip on this!)
In my opinion, after reading that you have the same problem, this is surely a BIOS issue. Like yourself, I had no issue with this with BIOS 49 or any earlier version.
Edit: I also have the DZ77GA-70K