"Floss" follows its own rule: after a short vowel in a one-syllable word, f, l, s, and z usually double at the end. Here's the full pattern, the word lists, and the handful of everyday words that break it.
The rule in one sentence
In a one-syllable word, right after a short vowel, the final letter doubles if it's f, l, s, or z. Compare cuff (short vowel + double f) with chief (long vowel sound, no doubling needed).
The everyday exceptions
Common short words that break the rule
A handful of very frequent words don't double despite ending in an FLOSS letter after a short vowel: of, if, us, yes, gas, bus, plus, this, has, is. These appear often enough in early reading that they're worth teaching explicitly as a short exception list rather than letting a child guess and get it wrong.
Where this fits in the bigger picture
The FLOSS rule is one of several spelling conventions covered in our 10 English spelling rules guide, and pairs naturally with practice on consonant blends since both focus on what happens at the end of short, simple words.