I have only ever seen the 'diss' spelling, perhaps your dictionary needs revising.

There aren't many single syllable verbs that end with an 'is' sound that doesn't end with a 'ss'. In fact, I think there are none. Outside of verbs, it's almost only names (and 'this'), so 'dis' would be a pretty rare way to spell a word in common use. I also think this is because it would appear natural to use the other new words derived from it, dissing and dissed, with the 'ss'. You don't need to use these forms with nouns etc. I think it's only normal for people to end up spelling it 'diss', especially as the original word it was derived from becomes less known.

That said, I am a second language English speaker and haven't even completed English as a second language in high school.

It's also now the dominant spelling, and can't really be considered "incorrect".