[AndrewCologne]

That’s normal, since –with the same initial CO setting kept– a warmer control pressure from the WUR makes the mixture result leaner, and therefore the duty cycle will rise to compensate this.
But here we’re still talking about constant pressure readings, correct? No pressure oscillation?
Because a non-constant system- or warm control pressure, and therefore a non-constant mixture, will cause surging.

Many years ago in Germany, there was a retrofit kit available that added a catalytic converter including lambda control to a 930.10/204 hp ROW engine so it could obtain Euro emissions classification here in Germany.
The lambda control was based on an oxygen sensor and a special ECU that regulated warm control pressure using a solenoid valve (similar to the frequency valve) in order to achieve lambda 1 in the mixture formation.