After upgrading splunk to 7.2.3, our netflow logs have a timestamp from 2015. We are using the newest versions of Netflow 3.8.8 and Netflow Optimizer 126.96.36.199.858.
Not much of a help, but I just updated to 7.2.3. and 3.8.8 and I don´t see any problems.
Just to give some kind of feedback
not sure where the issue is but changing the DATETIME_CONFIG to CURRENT from the default of auto fixed it, of course. Not sure why we have the issue in the first place though. Still researching.