A default value is only loaded when a value does NOT EXIST in the DB. What you have described is clearing the value, or selecting #fff. This is still a value and still EXISTS in the DB. THerefore the default value will not load.
The actually row in the DB must be completely removed from the DB for ACF to load in the default value.
If you require customization to this, please read the docs for the ‘acf/load_value’ filter
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