There is no EU-harmonised limit for melatonin in food supplements - each member state sets its own rule.
No Maltese national maximum was identified. Malta applies EU food law, and no national "other substances" maximum-level list could be located. That absence is the finding rather than a permission.
One specific residual risk is worth stating: Maltese medicines regulation is historically UK-influenced, and the UK treats melatonin as a prescription-only medicine at any dose. A divergent Maltese medicinal classification cannot be excluded on the evidence available.