Gurgling Toilet when someone takes a Bubble Bath

We have a downstairs toilet that gurgles whenever someone takes a bubble bath in the upstairs bath tub, and drains the water. Once it started filling up with soapy water.
I wonder what's causing this. Is there so much air in the bubbles that it gets past whatever prevents backflow?
It's a Toto Drake toilet.
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Probably the bubble stuff making big bubbles in the drain pipe and perhaps blocking the vent pipe.
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