Same issue on Ubuntu focal with linux 5.4.0-17-generic x86_64.

Purging plymouth then installing it back with all upstream dependencies does not change anything:
# apt purge plymouth
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following package was automatically installed and is no longer required:
  libplymouth5
Use 'apt autoremove' to remove it.
The following packages will be REMOVED:
  lightdm* plymouth* plymouth-label* plymouth-theme-spinner* plymouth-theme-ubuntu-gnome-logo* plymouth-theme-ubuntu-gnome-text* plymouth-theme-ubuntu-logo*
  plymouth-theme-ubuntu-text* slick-greeter*
0 upgraded, 0 newly installed, 9 to remove and 38 not upgraded.
1 not fully installed or removed.
After this operation, 4,028 kB disk space will be freed.
Do you want to continue? [Y/n] 
(Reading database ... 747060 files and directories currently installed.)
...

update-alternatives: warning: alternative /usr/share/plymouth/themes/ubuntu-gnome-logo/ubuntu-gnome-logo.plymouth (part of link group default.plymouth) doesn't exist; removi
ng from list of alternatives
update-alternatives: warning: /etc/alternatives/default.plymouth is dangling; it will be updated with best choice
update-alternatives: using /usr/share/plymouth/themes/bgrt/bgrt.plymouth to provide /usr/share/plymouth/themes/default.plymouth (default.plymouth) in auto mode
update-initramfs: deferring update (trigger activated)
...

# apt autoremove
...
# apt install lightdm plymouth plymouth-label plymouth-theme-spinner plymouth-theme-ubuntu-gnome-logo plymouth-theme-ubuntu-gnome-text plymouth-theme-ubuntu-logo plymouth-theme-ubuntu-text slick-greeter
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following additional packages will be installed:
  libplymouth5
Suggested packages:
  bindfs plymouth-themes
The following NEW packages will be installed:
  libplymouth5 lightdm plymouth plymouth-label plymouth-theme-spinner plymouth-theme-ubuntu-gnome-logo plymouth-theme-ubuntu-gnome-text plymouth-theme-ubuntu-logo
  plymouth-theme-ubuntu-text slick-greeter
0 upgraded, 10 newly installed, 0 to remove and 38 not upgraded.
Need to get 98.8 kB/896 kB of archives.
After this operation, 4,453 kB of additional disk space will be used.
Do you want to continue? [Y/n] 
Get:1 http://fr.archive.ubuntu.com/ubuntu focal/main amd64 libplymouth5 amd64 0.9.4git20200109-0ubuntu6 [98.8 kB]
Fetched 98.8 kB in 0s (426 kB/s)       
Preconfiguring packages ...
Selecting previously unselected package libplymouth5:amd64.
...
update-initramfs: Generating /boot/initrd.img-5.4.0-17-generic
cp: cannot create regular file '/var/tmp/mkinitramfs_K7OV2b/usr/share/plymouth/themes/spinner/watermark.png': No such file or directory
E: /usr/share/initramfs-tools/hooks/plymouth failed with return 1.
update-initramfs: failed for /boot/initrd.img-5.4.0-17-generic with 1.
dpkg: error processing package initramfs-tools (--configure):
 installed initramfs-tools package post-installation script subprocess returned error exit status 1
Errors were encountered while processing:
 initramfs-tools

# apt-file search /usr/share/plymouth/themes/spinner/watermark.png
#