There's clear guidelines on flex. It's not up to the manager how many days you come in, it's what days you need to come in. They are mandatory forcing full flex to come in 3 days min and your manager can make you come in on your wfh home if it's needed.
Soon the 3 day will be full 5 days, with the way they are laying down the pipework. They are easing you in but the carpet will be pulled, probably by summer when they realize flex is disturbing business and ppl have their own schedule that contradicts other teams and no "collaboration" is being done.