Why do some people's (last active) status disappear (and reappear) randomly on Facebook?

Anonymous
Sometimes, there are people whose status just disappear on Messenger/Facebook. Like, for example, there's someone (a friend from real life) whose status just randomly go from "Last active..." to... basically nothing. Sometimes, I just can't see when was the last time he went on Facebook. So, there are moments of the day when it shows up when was the last time that he was active, and then a few moments later during the same day (like, less than an hour later for example), it just disappears.

Sometimes, the status disappear for a few days (it depends on which contact). I find this pretty weird. I mean, if you turn off the "Last active..." setting, then turn it on, normally, it should show to the others that you are or have been online, right?

So that's why it makes me so confused.

And also, earlier today, it said that my crush went on messenger "an hour ago", but now, for no reason, it just doesn't show when she was last active (And I mean, I didn't send her anything recently, so she has pretty much no good reasons to "hide" from me).

There are even times when I can't see my best friend's status (even though he goes online regularly).
Updates
+1 y
PS: I just checked again, and it showed that my crush was online 5 hours ago (I couldn't see anything 10 minutes ago).

So yeah, is this just a bug/error?
Why do some people's (last active) status disappear (and reappear) randomly on Facebook?
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