I recently had three new toilets installed and two of the toilets were the same model. On one of the toilets the bowl fills normal but the water seeps to a low level. I have had a plumber out and he replaced the toilet and the same problem occurs. They checked the vent stacks several times and used a device with a camera hookup to explore the vent setup.
The plumbers are stumped as to what the problem might be. Let me also say that my old toilet did not experience this problem.

Could be defective toilet. Remove toilet, set up on saw horses, fill bowl with water. See if bowl leaks.

Thanks for the suggestion, we tried replacing the toilet with a new one. The plumber set the original toilet up in his shop with no problem. The replacement toilet still is losing water.

Thanks for the suggestion, we tried replacing the toilet with a new one. The plumber set the original toilet up in his shop with no problem. The replacement toilet still is losing water.

If there were no defects in the bowl the first thing I would check is the bathroom vent. If it's blocked then a suction would be set up every time you flushed or drained that could lower the bowl level. I'm surprised the plumber didn't inform you of this when he found no defects in the old bowl. Regards, Tom

Where exactly did the plumber insert the camera to check the vent? I see no other reasons than a clogged vent if the toilet is good.

Thanks for your replies. As it turns out the plumber had not hooked up the original toilet at his shop until my last inquiry. He experienced the same problem at his shop and my contractor is giving me an upgrade replacement. Thanks again. You guys really helped.

As it turns out the plumber had not hooked up the original toilet at his shop until my last inquiry. He experienced the same problem at his shop
Are you saying that both bowls were defective? Regards, Tom

Yes, as it turns out both toilets were defective.

Thanks for the update, Tom