Did you read this from the FAQ in the manual?
Why does the mixture behave strangely in the turbocharged version, and I cannot bind it to hardware controls?
Microsoft Flight Simulator’s turbocharger simulation has been significantly flawed for several generations. This aircraft has a custom turbocharger that fixes nearly all of these issues, and is much more realistic, as a result. To make these changes, the new “Input Event” system is used to intercept hardware and key-bindings for the mixture control axis. Please make sure that your hardware bindings are using the Key Events, such as “K:MIXTURE1_DECR_SMALL”, or “K:MIXTURE1_SET” to set the mixture, and NOT setting either “A:GENERAL ENG MIXTURE LEVER POSITION:1”, or “B:FUEL_Mixture_1_Set”. Alternatively, setting “L:BKSQ_MixtureLeverPosition_1” from 0-100 will also work to set the mixture axis.