I assume that you are using the MATLAB live editor for root locus plot. This is a known issue, it is reported to the developers and the concerned people are looking into it.
One known workaround is to:
1. Create the root locus plot with either 'rlocus' command or the 'rlocusplot' command from a MATLAB script file or the command line.
2. Then use data cursor to examine various points of the root locus plot.
Kiran Felix Robert