Whenever process instance is read (either direct call to getProcessInstance or through signal to continue with the process flow) it will read up all process variables regardless where they are stored. Next when process instance variable is changed it will trigger update in variable instance log. Though the variable instance log is only changed if the current value for process variable is different than the new one.

When using external data store for variables (pluggable variable persistence strategy) both variable copies (one known to process instance and second that is the new value) are exactly the same - it's because when we complete subprocess or task and use data output mapping to move variable from subprocess/task to process instance variable engine will load the variable from data store again which will result with exact same value being loaded so logic to check if old and new value of variable is different results in false and thus variable instance log is not updated.