How do I fix a bubbling toilet?

I have 2 toilets in our ranch style home. I have replaced both toilets about 5 years ago with water saving models. However, the master bath toilet often makes a bubbling sound and the water level drops to just within the depression within the bowl. Usually it corrects itself by next use.

It seems to happen very randomly without any other plumbing running at the same time.(no dishwasher running, no laundry, other toilet flushing, etc.)

Any ideas how to address this issue so that the toilet stops bubbling and maintains a consistent level?

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