It's about a variable:
I see that sometimes it is empty for some visitors. I checked myself both in normal mode and in incognito mode, this variable is not empty.
Question: an ordinary site visitor (a person) can have this variable empty in some natural case? Or is it still a sign of a robot or that a person is deliberately hiding it?
And a person can hide and may be a sign of a robot. But a robot can not only have this parameter, but also randomly change it, like all others, by the way, like a referrer, user agent, etc. If your task is to separate the user from the bot, the problem is actually much broader and it cannot be solved right away.