While getting this I also noticed that if, while the fsck is in process and plymouth is not hanged, I switch to vt1 and wait until I see an [ Ok ] (I don't know which boot step prints this as it's the only thing on vt1) and then switch to vt7 plymouth terminates correctly and the gdm login appears on vt7.
Here you are the backtrace of plymouthd.
While getting this I also noticed that if, while the fsck is in process and plymouth is not hanged, I switch to vt1 and wait until I see an [ Ok ] (I don't know which boot step prints this as it's the only thing on vt1) and then switch to vt7 plymouth terminates correctly and the gdm login appears on vt7.