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My experience I have seen the following:
1 The water pouring out of your tank into your bowl has an air gap between your lower jet at the bottom of your bowl and the tank.
Bowls that have many jets under the rim reduce this from happening. It generally happens to bowls with less jet under the rim.
The water being flushed through the main jet, at the bottom of the bowl can have air being pushed from the tank water and therefore causing a bubble.
If this problem just started to occur:
Then I would say your drain line is clogged or partially clogged.
Good Luck and If I can help answer more questions please ask.