That's why I asked earlier if your nandroid included boot partition; this should fix if you also included boot when restoring. If so, there may be other issues either with TWRP (as others indicated), or with system generally...I thought that when I was restoring with my backup, it was also restoring the boot to its pre-Magisk state, since I selected "boot" in TWRP when I created my backup. Am I wrong?
What's the proper way of getting this boot.img? I did some research and everywhere it says that to get the boot.img, you have to download the latest firmware for your phone model/carrier and extract it from those files.
I'd still try flashing boot image for reasons mentioned in official Installation Instructions:
In some custom recoveries, the installation may fail silently (it might look like success but in reality it bootloops). This is because the installer scripts cannot properly detect the correct device info or the recovery environment does not meet its expectation. If you face any issues, use the Patch Image method as it is guaranteed to work 100% of the time. Due to this reason, installing Magisk through custom recoveries on modern devices is no longer recommended. The custom recovery installation method exists purely for legacy support.
The proper way to obtain boot image is also in the instructions (nb. don't use latest, use image from whatever ROM you use currently).