Have you done the actual 3.2 update yet? Do you still have your Magisk 3.1 image?
- adb reboot bootloader
- fastboot boot old-magisk.img
- adb shell
- su
- dd if=/dev/block/by-name/boot of=/sdcard/boot.img
- Use Magisk.apk to modify /sdcard/boot.img
- adb pull /sdcard/new-magisk.img
- adb reboot bootloader
- fastboot boot new-magisk.img (just to double check)
- adb reboot bootloader
- fastboot flash boot new-magisk.img