I’m afraid that TWRP is only half the battle. As I have described here, there are also ways to root the FP3 that do not require TWRP.
Unfortunately, Magisk (v25.2) also seems to have a problem with the dynamic partitions, because the generated boot.img ends in a boot loop.
What about latest Magisk v26.1? Can anyone confirm that this release works again or not?
New Magic Mount Implementation
Magic Mount, the feature that make modules modify partitions, has gone through a major rewrite. The existing implementation doesn’t work well with OEMs injecting overlays into their system using overlayfs. The new implementation fundamentally changes how filesystem mirrors are created, giving us a more accurate clone of the unmodified filesystem.
Sorry for the confusion and posting in the “wrong” topic, but TWRP indeed was not involved.
I tried the way you mentioned but in step 3 the phone won’t start with the patched boot.img. Instead it instantly reboots and loads the unpatched stock boot.img from its internal storage.