Leaks can do unpredictable things.

Mouthpiece is a good thought, too - some of these older instruments are extremely sensitive to mouthpiece choice and can work much better with something closer to what they were designed to be used with.

If it's only the one note that's affected, I doubt it was designed to be used at significantly lower pitch. That would also be unusual for a 20th century instrument although some early 20th century instruments tuned slightly lower than 440.