Time jump when starting PTP service at boot

Hello,
I have noticed a strange behavior on my jetson xavier and I would like more information.
I am running Jetpack 4.6 and L4T 32.6.1.

I set up PTP time sync using linuxptp’s ptp4l and phc2sys. When running PTP sync manually it works fine.

Then I made them both run as service at boot. And here I get the clock jumping back and forth.

The solution was to make sure ptp4l waits for network manager to be fully initialized before starting, using the NetworkManager-wait-online.service

After=NetworkManager-wait-online.service
Requires=NetworkManager-wait-online.service

But now I would like to know what was causing the time jumps and chaging the clock. Do you have any idea ?

Thanks !

Could you share the steps/commands to reproduce this issue?