“Modern standby” is indeed the culprit in many cases, maybe even the primary one these days, but to my understanding it can still be a crapshoot on laptops that support S3 sleep since it’s up to the OS to detect that the lid has been closed and put the machine to sleep. This has been a problem for a very long time, since well before it became cool to pretend to be a smartphone and not actually sleep the machine.

There’s also wake on LAN which if enabled can rouse the machine from sleep after it’s successfully entered a sleep state.