Are you not aware of the different forms of English? There are several differences between American English and British English, “spelled” and “spelt” being one example.
Collins and Merriam-Webster are both American English dictionaries, and the Oxford English Dictionary is a British English dictionary.
Spelt is not proper English.
Edit: I retract Collins as a trustworthy source.
Of course it is!
Are you not aware of the different forms of English? There are several differences between American English and British English, “spelled” and “spelt” being one example.
Collins and Merriam-Webster are both American English dictionaries, and the Oxford English Dictionary is a British English dictionary.