Broken: Blender 2.83.5, blender-2.90.0-21cb6f09ffa8-windows64, blender-2.91.0-396d39c6b904-windows64
Short description of error
The original sampling presets in Cycles can't be deleted. However, it is possible to create presets of the same name. These presets become undeletable, because Blender assumes them to be the default presets.
Exact steps for others to reproduce the error
Create sampling preset with name "Preview" or "Final". Try deleting new presets.