Somehow fixed in v9.2.1 with no additional action from my side. Quite literally:
- Downloaded latest release from GitHub
- Launched, opened random book
- It works perfectly fine in 99.9% cases. The only way I can trigger the bug now is by finding an odd page which finishes with <hr> element and empty space AND the next page starts with the header, then 1-frame "blink" appears... Very rare occurrence indeed.