Basically, I thought, it was happening because I was changing CSS and something somewhere went wrong and I couldn’t find what’s precisely, so whatever. But today I saw this happening on demo site https://demo.getpublii.com/themes/mercury/3/ , well, guess it is a bug in a theme itself then.
It happens in Vivaldi browser version 3.6.2165.34 (Stable channel) (64-bit)
Also, in Opera app for Android, ver. 61.2.3076.56749
Also in Chrome 88.0.4324.104
Interestengly, this happens way more often if you access URL via speed dial button. Create a new button with URL of the demo, click the button around 10 times and it happens
It is annoying, nothing serious though. Maybe you can check it out.