I suppose David and Brodiemac1 use LDAP shared groups also. Can't suggest much. You can try latest Spark build, but it probably won't change anything. Looks more like a server issue.
You can also try Openfire 3.9.3, but unfortunatelly it has its own bugs. One of the LDAP related (groups disappearing after some time (
OF-830). Also some users experience memory leaks or ghost sessions issues (OF-829, OF-813). So, you should probably use it only if you want to test if it fixes this problem at least (on a test server or something).
Spark latest build http://bamboo.igniterealtime.org/artifact/SPARK-INSTALL4J/shared/build-665/Insta ll4j/spark_2_7_0_665.exe