I have identified the condition which this happens; in any segment, if there is a voice containing more notes than voice 1, the Theorytab fails to load properly within Hookpad, and does not load at all from the My Tabs page. This sounds like an out-of-bound error while creating some kind of note event array.
Thank you @HertzDevil
That was indeed the problem. I have a fix ready to go. It should be pushed live later today. For now, write your longest melody in voice 1