Okay, before posting this topic, I’ve already disabled the caching/optimizing plugins. After disabling and enabling all plugins, the problem seemed gone. But after some time, it came back. It took me some fiddling to find out that the problem was caused by Jetpack’s lazy image loading. I’ve disabled that and enabled the same function from Autoptimize, purged all caches several times and is looks okay now.
Thanks for the support!