Vent placed before P-Trap

Hello, thank you in advance for any insights. Long story short: Is there ANY situation where a vent should be BEFORE (upstream) the P-trap? Long story: Our professional plumbing company, with countless customers and excellent 5 star reviews, tied into a vent, going out the roof, BEFORE (upstream) a p-trap under the shower when he redid the plumbing for our guest bathroom. (We questioned specifically why the vent was before the trap before the work was done, we even had him draw us a picture, and we were assured that this was the way it should be done in our situation.) It solved the problem of our slow drains, but since then we smell a sewage-y/earthy smell coming from the shower drain every once in awhile. We can't pin point any specific cause, it's not every time we flush for example. We had the company send a different person out to inspect it yesterday, who checked all the seals and everything else that could possibly be causing the problem. He took photos and videos of the layout and sent them to his boss. Their solution is to send the same man who installed it out to replace the P-trap with a slightly smaller one at no cost to us. Should we allow the company to replace the P-trap or is the real problem that the vent is in the wrong place and we should cut our losses and find a new plumber?

post a picture.

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Here's a pic.

View attachment Vent Before P.pdf

no, hell no

Your plumber has no idea what he is doing. Make him correct it or call the building dept. and have his license taken from him.

Thank you both. I honestly don't know what's scarier/worse. Either all three men involved from this very experienced company in Los Angeles, including the owner, firmly believe that this is done perfectly or they lied repeatedly to our faces with incredible skill and are attempting to get out of fixing it. They have hundreds of 5 star reviews on Yelp. I just don't understand.

Me thinks your "plumbers" are not real licensed plumbers. That mess is unbelievable. Call the local jurisdiction and have them come out and inspect the work.

2012 upc code for california

905.2 Horizontal Drainage Pipe. Where vents connect to a horizontal drainage pipe, each vent pipe shall have its invert taken off above the drainage centerline of such pipe downstream of the trap being served.

call your local building department , ask to speak with the plumbing inspector
email that picture to him, or have him come to your house
not only was plumbing work preformed, it was done with out a permit pulled
the inspector will not like that AT ALL