Last Sales Activity (LSA) Recognises Messages for Customers but not Contacts

Hi all

We have enabled LSA (via Setup > Company > General Preferences and click the Custom Preferences subtab, the Last Sales Activity group etc) to track all of tasks, events, phone calls, notes, messages and marketing campaigns.

However whilst it is working fully for customers (ie companies) it is not completely working for contacts (ie people), see example:

I have a customer that only has one contact.  When I look at a customer it tells me (correctly) that the LSA for that customer was a message on 21 Sept 2018.  However when I look at the contact, it lists all the messages (including the one of 21 Sept), however in the LSA field it indicates an event of 21 August 2018, ie the contact's LSA does not seem to be picking up messages.