using $R is not returning random character in filename

When using $R, $RRRRR or $RRRR to generate random chars for the filename, the resulting file name has the $R being replaced with the current system time, and the remaining Rs are used as-is.

An example would be:

shutter -f -o '~/screencaps/$RRRR.png'

results in a file named '23:45RRR.png' instead of the expected 'asdf.png' or similar. The man page lists four as an explicit example "$R = random char (e.g. $RRRR = ag4r)". I'm not sure if this is a system configuration issue, or something that should be submitted as a bug instead.

Running Mint 18.3 with Linux 4.13.0-37 on a ThinkPad W540, and shutter version 0.93.1 revision 1278.