When a pickup measure is present, Hookpad treats the selected note as two separate notes, both when playing and when exporting the tab, even though only one note is present:
Although not shown here, the same thing also happens for chords spanning across the pickup measure. This breaks any tabs that use syncopation at the pickup. Please merge them back into a single note.
A similar argument can be made about notes spanning across a key change (that however requires extra effort to make the note refer to the same pitch before and after the modulation, which Hookpad doesn’t at the moment).