Thank you for quick reply @Nikko
In the past time, I had one time experience that Time Zone mismatch was a root cause for not seeing logs on Panorama. This is reason why I sent you that link, however after you confirmed that it actually works in your deployment, I tried to reproduce it by changing Time Zone on one of my Firewall. The result is: logs are still being displayed in Panorama.
To answer your question, I believe that log time stamp on Panorama side will be from Panorama's time. This is what I see in my case after I have configured different Time Zone on Firewall side.
You can verify it from CLI. The below commands will show time of the logs when it is being sent to Panorama and received by Panorama.
Firewall: show logging-status
Panorama: show logging-status device <Firewall Serial Number>