When using the OPTION FAST SQL Server passed the criteria as a fixed value while without it, it would use a variable because of a conversion.
Then I remembered the following article which states a list of situations where auto-parameterization is not happening and one of it is:
I was passing a uniqueidentifier between single quotes so apparently the auto-parameterization is storing it as a character variable in the cache.
A statement with query hints specified using the OPTION clause.
Another mystery solved :-)