There isn't a fix for this issue yet.
The workaround is to edit the rule definition in xml format from a MonitoringPolicies.xml file and import it back in the FMS.
Use this command from [FMS_HOME]\bin ([FMS_HOME]/bin on linux) and enter the password when prompted.
fglcmd.bat -usr myuser -pwd -cmd util:configexport -f "D:\Quest\MonitoringPolicies.xml"
./fglcmd.sh -usr myuser -pwd -cmd util:configexport -f "/tmp/MonitoringPolicies.xml"
Clean up contents from the resulting file but keeping the top two lines, the last line (highlighted in green on the screenshot below) as well as the contents of the affected rule (element contained between severity-family tags) in the XML file.
After cleaning up the file, copy the severity (element contained between conditional-severity tags) and adjust the level of the severity accordingly. In the example below, we're taking the Critical severity (Level 3) and copying as Warning (Level 2). Take notice that the id was modified to a unique value. The Critical severity is highlighted in turquoise and the Warning severity is highlighted in purple.
After this change, save the file and import it back into Foglight using the command line as before.
fglcmd.bat -usr myuser -pwd -cmd util:configimport -f "D:\Quest\UpdatedRule.xml"
./fglcmd.sh -usr myuser -pwd -cmd util:configimport -f "/tmp/UpdatedRule.xml"
More information about the format of the file on KB 46240.