I think I incorrectly attributed the warnings to the Plymouth package. I now think this is an issue with update-initramfs. I saw the warnings again when manually running initramfs-tools.
My apologies if I got the package wrong (again). ----- # apt-cache show initramfs-tools Package: initramfs-tools Version: 0.142 Installed-Size: 113 Maintainer: Debian kernel team <debian-ker...@lists.debian.org> Architecture: all Provides: linux-initramfs-tool ... Section: utils Priority: optional Filename: pool/main/i/initramfs-tools/initramfs-tools_0.142_all.deb Size: 72864 MD5sum: b00b1c4ad31046ed1f80852bf20161f2 SHA256: dd1b867f0000d967471b0f775fdb3b4a534d4ae70bdb56a93b1ffcf60965813b
