How "update contact history" work internally?

[Jidong]

We use RTDM 6.5 , we have a decision flow have two sub-diagram,  three reply nodes checked "update contact history" box, about a weeks before, we can see three records generated in contact history table for every communication.

Two days before, we find that there are only two records generated in contact history table for sub-diagram, the record for main-diagram lost, we change nothing for this decision flow, so we suspect there are something wrong with testing database, so we restore the testing database to earlier version.

Now, we find there are only one record generated for main-diagram , record lost for sub-diagram, how  "update contact history" work internally? how can we debug this kind of problem? thanks.