This is rather curious because the previous generated code should result in compiler errors. The expanded code does not error, but manually copy+pasting the generated code reveals the compiler error as expected:

error[E0170]: pattern binding `V1Alpha1` is named the same as one of the variants of the type `FooVersion`
   --> crates/stackable-versioned-macros/tests/test.rs:324:13
    |
324 |             V1Alpha1 => "v1alpha1",
    |             ^^^^^^^^ help: to match on the variant, qualify the path: `FooVersion::V1Alpha1`
    |
    = note: `#[deny(bindings_with_variant_name)]` on by default

The incorrect match resulted in as_str to always return the earliest version (usually v1alpha1). This resulted in the wrong version to be marked as stored when calling Foo::merged_crd.