Spanish (sin) - likely re-borrowed, otherwise it would be *sen
Catalan (sense), Occitan (sense) and French (sans) - partially mixed “sine” with “absens” (absent, away, missing), so it got a random -s out of nowhere
Italian (senza) - similar to the above with “absentia” (absence, awayness). -tia → -za is regular in IT.
Not surprised - Latin “sine” without became a mess in the descendants, that’s why they don’t resemble each other much: