Funnily enough, I have a similar problem when importing an .xml file from Sibelius - when I import, the guitar tab is totally out of joint and literally off the scales. If I hadn't searched for a solution, I wouldn't have found your problem, and thereby the temporary, yet excellent solution of resetting tab numbers. Prior to that I was adjusting tabs note by note and it is disheartening!
I love the overall appearance of Notion compared with Sibelius but there are a few clunky things about it, such as the above...