Turning off smart group cache causes database table size explosion

Due to the size and complexity of our smart groups, we have been having trouble with the group contact caching and so decided to switch it off. We did this by going to Administer/Customize Data and Screens/Search Preferences and setting "Smart group cache timeout" to zero.

However, the effect of this was for the code to continue to insert results into the group cache but not remove any. This led to an explosion in the size of the group contact cache table causing critical systems issues.

The code in CRM/BAO/GroupContactCache.php appears to intend to always truncate the civicrm_group_contact_cache table if this setting is zero, but this doesn't seem to be happening.