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Hello, my name is***** am a plumbing contractor. Your main line is probably partially backed mainline past the snakes reach. When the water builds high enough it starts a siphon and sucks the bowl clear because the vent in full of water and can't release the air lock. Do you have cleanouts outside that you know of?
This could also be caused by a toilet that does not start a good siphon at the beginning of the flush. In the bottom of the you can usually see a 1/2" to 3/4" hole pointing into the toilet drain. At the beginning of the flush does water gush out of that hole?