FWIW if this actually happens it's more likely the act of sending the message has made the server realise the user is no longer online.

It's inefficient borderline impractical for a server to send a frequent ping to every single user to check they're still online, usually it waits for them to perform an action within a certain timeframe and if they don't then either send the ping to see or just decide they're offline/away.

If you send a message that doubles as a ping to check

Me: Wanna play a game?

Op: Sorry, I have to get some stuff done tonight

OP is playing Codblops 4: Extra Vinegar

Well yeah its hard to focus on erotica when a friend starts texting you silly billy

Sorry friend, I just got online to recharge my social batteries, not drain them further when they're already near empty.

This except it’s my entire discord server. Everyone chatting away and as soon as I utter one word everyone leaves.