When a ScriptedImporter asset becomes orphaned (the ScriptedImporter type disappears), it changes to a DefaultImporter asset (*), but some state seems to bind it to the old importer type:
- restoring the old importer type reimports the asset correctly
- adding a new type of scripted importer to handle the extension instead, results in:
> Asset import failed, "Assets/a.blah" > Scripted importer instance could not be created!

EXPECTING: Assigned importer has changed since last import, reseting import settings to defaults for asset 'Assets/a.blah': <unknown> -> Assembly-CSharp-Editor-testable::MockImporterWithObjectProperty.

NB:
- reimporting the asset manually at this point (or incrementing importer version to force reimport) fixes the problem
- swapping the importer type to a different one without a Refresh in-between doesn't show the problem

(*) PR https://ono.unity3d.com/unity/unity/pull-request/80725/_/assetpipeline/change-behaviour-when-scripted-importers-disappear makes this behaviour more visible as it fixes a problem that was previously hiding it (previously the asset was not being reimported automatically by the DefaultImporter (unless manually forced by the user) and switching the scripted importer when the asset is(/seems) still imported by another scripted importer works just fine.