Problem with traditional cleaning methods
What is causing the problem is a build up of uric salt crystals and sludge in the pipework and urinal components.
This provides a breeding ground for airborne bacteria which feed and produce foul smells.
The deposits are virtually impossible to remove with water and everyday cleaners which is why powerful chemical cleaners are necessary to shift them.
This cleaning is only a temporary fix as with reduced flush volumes the deposits return even faster creating a cycle which results in people often abandoning their water saving goals.