Tenant says something stinks every time they flush toilet

[Joe S.]

So the tenant has filled out a work request with the Property Manager. The tenant is saying that something smells every time they flush the toilet. These are the same tenants that filled out a work order some months back because they said they killed three spiders. 
The vendor that the PM sent is suggesting me change both of the Toilets which is about a year and a half old. The estimation is $800. My concern is after the new toilets are installed somethings gonna smell every time they go to the toilet still..😱😱

 Has anyone ever dealt with something of this nature that could advise whether there is a real issue and a way to fix it or whether it’s just the tenants that is the issue?

[Nathan Gesner]

Quote from @Joe S.:

The vendor that the PM sent is suggesting me change both of the Toilets...

And your PM accepted this?

When someone tells me to replace a toilet, I need some justification. How is the toilet causing the odor? And when a plumber tells me it's going to cost $800 to replace two toilets, I'll ask him if that includes cleaning up after me when I pi$$ my pants from laughing so hard. 

I've never had this happen once with hundreds of rentals and thousands of tenants. How is it happening with two new toilets in the same house?