Save-only error on newly registered file type with custom package

@maucejo 's https://github.com/maucejo/UFFFiles.jl was recently merged into the FileIO registry.
For my work, I have a custom package that handles similar files.

To handle the load / save logic, so far, I used the following in my package.

function load(f::File{format"UNV"})
    ...
end

function save(f::File{format"UNV"}, data)
    ...
end

detect_unv(io) = endswith(io.name, ".unv>") || endswith(io.name, ".uff>")

function __init__()
    add_format(format"UNV", detect_unv, [".unv", ".uff"], [MyPackage])
end

Since 1.18, which is the introduction of UFFFiles in the registry, my tests fail for save, but load works fine.

ERROR: ArgumentError: Package UFFFiles [20c5726e-8372-4c34-be2c-190a5a70d483] is required but does not seem to be installed

I could have expected that both success (as in previous FileIO versions) or fail, but I don't understand a partial fail.

Any ideas what could be the reason ?

I've attempted to make a MWE, but in a module, not totally a package, so the init-equivalent step may not be relevant as such.

using Pkg
Pkg.activate("TestFileIO")
Pkg.add(name="FileIO", version="1.17")

unvname = joinpath(tempdir(), "test.unv")
write(unvname, "test")

module TempFileIO
using FileIO

function load(f::File{FileIO.format"UNV"})
    open(f, "r") do s
        String(take!(s.io))
    end
end

function save(f::File{FileIO.format"UNV"}, data)
    open(f, "w") do s
        write(s.io, data)
    end
end

detect_unv(io) = endswith(io.name, ".unv>") || endswith(io.name, ".uff>")

add_loader(format"UNV", detect_unv, [".unv", ".uff"], [TempFileIO])
end

using .TempFileIO

using FileIO

load(unvname) # Works regardless of version
save(unvname, "test") # Fails in 1.18+ since UFF is registered

Reproduce the failure with the provided TestFileIO environment and the module's load, save, detect_unv, and add_loader entry points. Compare resolution before and after registering UFFFiles, then determine why save requires UFFFiles while load succeeds. Done means the behavior is consistent and the provided MWE succeeds or reports a clear, intentional error.