My chip's datacode is 1139.
I don't have an oscillo to look at the signals so I don't know why/how the RX0 pin works with or without the RC in front of it but can say for sure that without it I cannot program the 1284P via serial using the bootloader.
Of course, no problems using the ISP programming.
A friend of mine has a datacode of 12xx and he is experiencing that bug in a stronger manner than me, because he isn't able to program the 1284P using the bootloader nor with nor without the RC filter.