Hello Scott,
This issue was resolved in Build 6919 (Dec 11, 2018) "Fixed: Scenario in which users could lose signature mappings." I'm afraid I'm not sure whether this is a retroactive fix or if users will need to re-map their signatures. However, it shouldn't occur on future upgrades.
Andrea Free
SmarterTools Inc.
877-357-6278
www.smartertools.com