Actually, it's correct, if not what most people would want. Then again,
XL is not a word processor. As documented in Help:

Proper

Capitalizes the first letter in a text string and any other letters in text
that follow any character other than a letter. Converts all other letters to
lowercase letters.


In article ,
Josh Ross <Josh wrote:

The proper function incorrectly capitalizes the letter after an apostrophe.