It depends on how you're thinking about modal- from the perspective of the piece (or section of a piece) as a whole, or at that exact moment.
Looking at only the exact instance in which the leading tone is raised, no, it is not in the mode. However, looking at it in context, the section can still be considered modal, with the leading tone providing tension (it's a tritone away from the 7th of the V7 chord) that most likely resolves to a I or I7 chord.
As you said, this is common in tonal music in a minor key. It is also common in modal music. Take a look at Cannonball Adderley's solo on So What, which alternates between B dorian and C dorian. He plays A# over B dorian and B natural over C dorian several times, along with other notes that are outside the mode, but it it is still considered a modal piece of music. From the wikipedia article on modal jazz, "Kind of Blue [the album with So What] is an exploration of the possibilities of modal jazz."