Filter doesn't make a mistake, ignoring the space is known but filter doesn't warn you for sentences that doesn't have swear. I copy pasted your same sentence through my alts and its good for me so not a bug. You must have typed something containing it first time which made it show and you could have modified it because if its a bug, then it works for me.
If this happens, maybe try refreshing and check. If it keeps happening again and again even after you checked your sentences then report us with the sentence so we can investigate.
Thanks, if this happens again. Then let us know with proof of ss or the sentences.