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Might want to have a read up on control theory....
The only mechanism that it has to try and keep you at iso power is by varying resistance - if your cadence drops then the torque must increase to compensate.
Zwift has a handy feature that will drop you out of erg mode if your power drops too low from the target and allow you to spin down then as you bring power back up it re enables erg mode for you.
Also has a button you can hit on the mobile app to manually toggle erg mode on and off
Stick to building bridges.