I've been banging my head against the wall with this same issue for the last week. Pretty sure it's something Microsoft broke with a recent update, but I can't find any current discussion on it.
In my case the settings are set via group policy, Endpoint Manager is only managing compliance. But the result is the same, I've checked and double-checked GP settings as well as local settings on multiple device and no matter what settings I adjust, devices lock after 15 minutes.
Technically I can make them lock sooner than 15 minutes, but not longer than. It's maddening.