Why would that be grammatically incorrect? I was always taught that if the name ends with an "s" you omit the "s" after the apostrophe.

I looked it up, and it's not quite that simple.

https://www.merriam-webster.com/grammar/what-happens-to-names-when-we-make-them-plural-or-possessive

For names that end in an s or z sound, though, you can either add -'s or just an apostrophe. Going with -'s is the more common choice

Ok, so it's kinda optional in most cases, but reading on:

For classical and biblical names there are other rules. For names ending in s or es and having two or more syllables, you usually just add an apostrophe. If the name is only one syllable, add -'s.

• Socrates' students
• Ramses' kingdom
• Amos' prophecy
• Zeus's warnings

The names Jesus and Moses are always made possessive with the apostrophe alone:

• Jesus' disciples
• Moses' law

So pretty obscure stuff, but the bottom line is that the title is the only correct way to write it according to M-W at least.