I don't think MS has ever shared the reasons.
But Debra Dalgleish does share some workarounds:
http://contextures.com/xlautofilter02.html#String
Kieran H wrote:
To all,
Does the custom option in Autofilter have a limit on the length of
string/field it is prepared to search.
I have evidence that this the case but would like to understand the
reasons
Cheers
Kieran
--
Dave Peterson