The "R" Registers are in effect file registers, these are not enabled by default, usually, this file is configured & the PLC parameters are then downloaded, The file is created on first boot, these are retentive (provided the battery is good, I have never had a problem with them, is your battery ok, I have known (at least on the A series) that even without a battery it retained it's program for over a week.
I would check that the R register file has been set in the PLC parameters i.e. upload it to your PC if not already done so, the only reason I could think it is not working is that for some reason the parameter file has been downloaded that does not contain the file details, it's only a guess but cannot think of any other reason for it not to populate the registers.
I suppose it is possible someone has uploaded just the PLC code into a new file but not the PLC Parameters, but then downloaded it including the parameter file that does not contain the information to generate the file, never tried it so this is also a guess, powering down would mean it may have loaded the PLC Parameter file that did not generate or knows about the "R" file on a re-boot. Cannot remember if it throws an error if there is no file, if I get a chance I will try it on a Q6 I have.