Premise: I own a Oneplus 3 with OxygenOS OB28.
The SafetyNet check failed, so I tried to both reinstall OxygenOS OB28 with Magisk itself.
The result for a long while was an unbootable device with the dm-verity warning at every boot (even in recovery mode).
Until I decided, as partial workaround, to reinstall Magisk v15.0 and v15.1 in that order, then wipe cache and reboot.
The dm-verity screen is still shown, but device now boot properly.
@topjohnwu please note that, according to recovery output, only Magisk v15.0 recognizes properly dm-verity and try to patch fstab.
The dm-verity error persists even by reinstalling firmware, even by installing Magisk v15.0, even by upgrading to Magisk v15.2.
It means I've to lock and unlock bootloader, to fix them... but I prefer to think there's an easier solution.