The shower drain in the upstairs master bath makes a gurgling sound when the toilet is flushed. There is no similar issue in the other upstairs bath or the half bath downstairs. The shower itself drains fine, as does the sink. The toilet flushes ok, other than the gurgling sound. I had a plumber come out and after checking the drain and the vent pipe, he found no problem. He said when a renovation was made, the vents were connected up in a different / "wrong" order. The renovation in the master bath was over 4 years ago, and this problem just started. He said the gurgling was no problem, and wouldn't be as long as the shower was in regular use and maintained water in the P trap. I had never heard about vents needing to be hooked up in a certain order, and I'm wondering if that makes sense.

it makes sense to me.

Thanks. What makes the order wrong? Is it out-of-code? How does it create gurgling? Is the gurgling the draining water from the toilet sucking on the bath's trap?

probably is..

I'm guessing the shower has a flat vent. that vent has clogged as all flat vents do over time.
this is only an educated guess.