Hmm, it could be a regional thing (or generational, I suppose), but I've never heard 'Sean' pronounced as anything but 'shorn' / 'Shawn'. However, your comment piqued my curiosity, so I read https://en.wikipedia.org/wiki/Sean and also asked ChatGPT (since my knowledge of IPA pronunciation isn't very good). It seems to largely agree with me (but concurs that the pronounciation you suggested is an edge case):

"For the overwhelmingly vast majority of people named Sean, the pronunciation is:

This pronunciation comes from the Irish name Seán, the Irish equivalent of John. In Irish, the combination “ea” is pronounced much like the “aw” in law, and the accented á indicates a long vowel.

That said, there are a few exceptions:

• “Seen” (/siːn/): Very uncommon, but some people with the spelling Sean deliberately pronounce it this way, especially outside Ireland or in families that adopted a unique pronunciation.
• “Sheen” (/ʃiːn/): Extremely rare as a first name. More often this is the pronunciation of the Irish surname Sheehan or the English surname Martin Sheen (whose stage surname is pronounced “Sheen”), not Sean.
• Regional accents: In some accents, “Shawn” may sound slightly closer to “Shorn” because of vowel changes or the presence (or absence) of the cot–caught merger. This is an accent difference rather than a different pronunciation of the name.

So if you encounter someone named Sean and have no other information, “Shawn” is by far the safest assumption. Pronouncing it as “Seen” or anything else would be unusual enough that the person would almost certainly expect to correct people."