Hm indeed a bit strange. Tbh I cannot tell why you cannot restore the previous image. My first guess would be that you rooted the device a bit differently and now the image to restore is not available. 
Maybe @hirnsushi has an idea?
Normally CalyxOS will tell you when it cannot install the Update (at least his was the case for me when I had it on my FP4). So you can at least try to just go to
Settings → System → System updates
and download and install the OTA Update.
When it has issues with root it will fail to install the update and tell you.
If it works I would just install magisk to inactive slot as described before reboot.
If you missed that - just root again 
- I still recommend the method from the first post (the one with only booting from a magisk patched image and then install magisk via app): How to root the FP5 and OTA update - at your own risk
Can you tell (if you remember) if this was the way you rooted the first time?