I am assuming you are using the BIX version found in the additional products folder of the Pega 7.1.9 distribution image (and not an older / newer one). The highest ruleset version for Pega-BIX in Pega 7.1.9 should be 07-10-19.
Use Last Updated Time as Start For classes that include the property pxCommitDateTime, select this check box to extract all data that was created or updated since the last time the Extract rule was run. This field is not relevant until the Extract rule has run at least once. Note: If you select the incremental extraction option, a filter condition is automatically included during an extract that restricts extraction to class instances where the pxCommitDateTime property value is greater than or equal to the last date and time when the extract ran. pxCommitDateTime is automatically set for each class instance when it is changed in the database by the Pega 7 Platform database regardless of how the data was created or changed (interactively by an end user, programmatically by an activity, through an import, or by any other means). This condition is added to any others you specify on this tab. Sometimes the pxCommitDateTime property may need to be added as a database column to the class table. For example, it is not added automatically on upgrades. An extract using the incremental extraction property will fail if this column does not exist in the class table. Also note that when incremental extraction is performed, class instances with NULL values of the pxCommitDateTime property will be skipped and when using the -c command line option to extract from child classes, those child classes whose class tables do not include the pxCommitDateTime property will be skipped. Do not select this check box if you plan to run the Extract rule using the command-line and intend to use the -d, -D, -u, and -U parameters. This is because these parameters do not override the check box setting , which can result in unanticipated results from a command-line extract.
PRPC table gets populated for the pxCommitDateTime, however, the values are not extracted (all values null for pxCommitDateTime) in the target database. I am planning to use agent to schedule the extraction, do I need to be concerned?
If you're not concerned with the value of pxCommitDateTime in the target database, then you probably don't need to worry about it. If this is a piece of data that you are required to track, then please raise an SR for us to look into it. As I said, I was able to reproduce.
Note that you can do a Property-Set of the pxUpdateDateTime value in an activity / data transform and that value could be in the past or future and when the instance is saved, the value set is persisted. The same is not the case with pxCommitDateTime. pxCommitDateTime is set by the platform at the time of commit to the database.