Why YSK?
The first person who typed “should of” probably heard of it in real life that was meant to be “should’ve”, they typed “should of” online and readers thought that it’s grammatically correct to say “should of” which is in fact wrong and it became widespread throughout the years on Reddit.
I hope something could start to change.
Yes, English spelling is very misleading.
That would be a mistake in all dialects of English. It is always pronounced with a /v/ sound and the vowel is a schwa. 've is also a schwa plus /v/.