Re: Firmware 4.5.2 QC35 ii
I also sticked to 4.5.2 and just downloaded bose connect 9.0.0 For me no change in behavior between high/low/off setting wrt ANC after using the new bose connect app.
I checked the difference between low and high by creating a windy environment (fonh or fan forcing air around my head) and the low settting has less issues with the wind as what the bose connect app explains in the description at the low setting.
the only difference to your situation is that i first deleted the bose connect app and then installed it new from the app store .
I have a QC35ii series AZ, ios 12.4 on a iphone 6s