After you incorporate in a standby, it may take several tens of seconds before the first extraction file begins to be generated on the plugged in standby. Extract files generated on the primary side during this time are not generated on the standby side. In other words, redundancy of userlog extraction files will not start for a while after you make an instance redundant.

On the standby side, extraction begins after all transactions, such as those in progress at the time of inclusion, have completed. Note that if there are very long transactions, the non-redundant section of such userlogs will be lengthened accordingly.

The reason for this behavior is that making the right extract file requires WAL from the oldest to the latest of all userlogs inserted by all transactions that were running at the time the installation was complete, which is impractical. You have to go back through the archive file to get it, and you don't know how far back you have to go.