Quick answer: Godot 4 custom ResourceFormatSaver not running on assets created via bulk import? Saver registration happens at startup; bulk creates after may bypass - call AddResourceFormatSaver explicitly.
Tool creates 50 .item resources; custom saver should run; none triggered.
Register at startup
EditorPlugin's _EnterTree: register saver. Always available.
Or call save_resource explicitly
Custom save loop calls saver per resource. Bypasses registration ambiguity.
Audit saver coverage
Each format's saver registered. Document.
“Saver registration timing matters. Bulk operations need pre-registration.”
If your project has custom savers, the startup registration is the safe pattern. Document.