Because you need to wipe in order to not have issues.Just to clarify some things. I was on the Nov 5 update using Magisks 2.5.2-delta-5. I then removed all the Magisks delta modules and sideloaded the Dec 5 OTA. I then re-rooted using Magisks 2.5.2-delta-5 again. All is working in that setup. I never attempted or had the 32bit setup in the Nov 5 build.
I attempted to enable the 32bit in the Dec 5 build using the following method.
1. Removed all magisks modules, uninstalled magisks app.
2. Flash original init_boot.img from the Dec 5 build.
3. Rebooted into phone and installed your modified magisks delta.
4. Patched original init_boot.img from the Dec 5 build using your modified magisks delta.
5. Flashed 32bit modified init_boot.img.
6. Rebooted into boot_loop.
To get back up and running I then flashed original init_boot.img then re-rooted using the Magisks 2.5.2-delta-5 to patch original init_boot.img and reflash.
All is working using the above steps. Not sure what is going wrong.
If you are sure that you have the patched Magisk init_boot installed on the phone then simply just run
fastboot -wto wipe your storage.
Generally there is limited success with not wiping, some 32-bit apps won't work properly because some libraries aren't configured properly when Android doesn't get to freshly set things up with 32-bit support from the start.