I am currently editing a scientific text in which the term "machine learning" (ML) appears several times with a hyphen (i.e., "machine-learning") and several times without. Are there any significant differences between these forms? As far as I understand, ML can either (1) represent a field of research or (2) a descriptor of methods/algorithms. Examples:
(1) Machine learning is a scientific field that studies the theoretical and practical aspects of programs that feature intelligent behavior.
or
(2) We experimented with several machine-learning methods.
At first, I thought that this difference in meaning between (1) and (2) can explain hyphenation. However, the selection of hyphenation in the text I am editing seems to be completely random and unrelated to the above. Thus, should I stick to one form (i.e., with or without hyphen, and which to choose?) for consistency or is there a related rule of grammar that I can follow?