I don't use LastPass so I can't give you specific instructions. But you should be able to disable its autofill functionality on specific websites that it has trouble with, while still taking advantage of it on nearly every other website. It may also be possible to manually fix the issue specifically for the Bittrex site so that you can even still use autofill on its website without having LastPass autofill in the wrong boxes.

This link may be helpful:
https://lastpass.com/support.php?cmd=getfeaturefaq&feature=feataure_4