Thanks Tom,
The symptom happens on iOS simulator and my iPhone XS.
After a few hours diagnosing, I found the issue to be caused by my caching plugin (WP-Rocket) which was disabled when you inspected.
For future readers, it was caused by enabling lazyload. Disabling lazyload consistently rectified the symptom for me.
My assumption is that the sticky nav logo image didn’t load until scroll. That’s why scrolling up again fixed it.
Thanks for the great theme Tom.