I think they made a mistake calling it a heat map. If it turned on then f8 twice should turn on heat and current chunk for that heat ive only ever used it in sp and mp i personally have no idea if this works on a server and since i dont use mods i have no idea if something you have installed is interfering with it the only thing i can suggest is this.

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I dont know anything about a heat map display only glanced at the console and dev options from time to time but that does sound like a mod.