With FCPX 10.4.7 on the latest Mojave 10.14.6, I am not seeing any issue with SD cards importing on multiple card readers. You can try a couple things. Make sure you have the latest Pro Video Formats 2.1.1 installed (this was just updated about a week ago). Secondly, if you haven't already, launch FCPX while holding down OPTION and COMMAND keys and choose to delete preferences. This can be beneficial the first time you run FCPX after a version update. See if that helps.