There is no EU-harmonised rule for melatonin. It sits outside the cannabinoid framework entirely and is governed nationally, with limits ranging from 0.3 mg in Belgium to 2 mg in Spain and no limit at all in several member states. The one EU-level figure is the authorised health claim in Regulation (EU) 432/2012, conditional on 1 mg per quantified portion.
Luxembourg is unusual in having no list of its own. Its food safety authority states that there is no positive list in Luxembourg for plants and other substances that may be used in manufacturing food supplements, and that the lists established in other Member States serve as the reference for the competent authorities. Melatonin is an other substance in that sense, so Luxembourg sets no national maximum for it and no Luxembourg decision reclassifying it as a medicine was identified.
Which reference applies is unresolved, and it matters: Luxembourg's immediate neighbours diverge, with Belgium at 0.3 mg per day and France permitting below 2 mg.