Thinking out loud (and I know that's dangerous): there's no pressure
in a drain line. They are designed to run at atmospheric pressure.

So there is no way a bubble gets forced backward through a trap unless
something temporarily increases the pressure.

My guess would be nothing wrong with that toilet, but something wrong
elsewhere in the system. Perhaps your wash machine line, dishwasher,
etc. Something else is producing system pressure that doesn't escape
through the roof vent and goes to your toilet instead.