Logminer cannot always populate all the fields of the v$logmnr_contents this is because the redo may/may not have all the information that we need for every column.

Fix
Enable supplemental logging on the database following Note 186150.1.

Refer

Article-ID: Note 110301.1

Title: Column USERNAME and SESSION_INFO are NULL in V$LOGMNR_CONTENTS

结论：Enable supplemental logging

The redo stream does not contain the USERNAME and SESSION_INFO data for every transaction. This information is only stored for the first transaction executed in the user’s session. So in order to be able to see this information in V$LOGMNR_CONTENTS, all the redo generated during the entire session must be added to the mining session. Should this not be done, then the USERNAME and SESSION_INFO columns will remain empty.