My father had a toilet replaced in his downstairs powder room recently. He noticed that the water in the toilet bowl dropped overnight. When he flushed the downstairs toilet, the water level in the bowl went back to normal.
I had him flush the upstairs bathroom toilet while I watched the one downstairs. When he did this, I noticed the water level in the downstairs toilet dropped. I suggested that there may be an obstruction in the vent stack and that flushing the upstairs toilet was causing the suctioning of the water from the downstairs toilet.
My father worked with the plumber that installed the new toilet and a contractor that went on the roof and cleaned the vent stacks. After the vent stack cleaning, flushing the upstairs toilet still suctions water from the downstairs toilet.
Looking for advice on what to try next.