In Merriam-Webster, our dictionary of choice, the hyphenated compound noun “decision-making” appears as such, so it's always hyphenated. (Most adjective forms, on the other hand, can be left open after a noun, even if they are listed in the dictionary with a hyphen.)

When should decision-making be hyphenated?

Traditionally, we would hyphenate decision-making when the two words acted as a unit with an adjectival meaning, e.g. decision-making paper. We would not hyphenate decision making in noun form, i.e. a noun+ gerund (noun derived from a verb). The decision-making process lasted for weeks.

Is decision-making hyphenated in AP style?

decision-maker, decision-making: Hyphenated in AP usage. default: Failure to meet financial obligations.