I think the mistake might come from a lack of reading. The contraction “should’ve” is often used in speech, which might be mistakenly written as “should of” if you don’t read a lot and see it written properly all the time. We’ve mostly lost the voiced quality of “v” at the ends of words like that, so it’s basically pronounced “should-uf” in American English.
White flag? Have Russian invaders been flying it falsely, or does Ukraine just not care? (Legit question, I haven’t heard about this. I stand with Ukraine.)