What version of RME are you using? I tested RME 4.0.5 (i.e. LMS 2.6), and I cannot reproduce the problem you describe.
In order for this to work, you must change the snmp-server location on the device, then run a new inventory collection job for that device in RME. Only after the inventory collection is performed will RME detect that the sysLocation has changed, and add the device to the group.
Of course, the group must be configured with Automatic membership dates as opposed to "Only Upon User Request".