Hi Jeff and welcome to the forum.
The Line() function has fairly limited application and simply returns the line of the page on which a record appears. Typical usage would be to combine it with a Page() function calculated field for some sort of audit purpose or one or two more obscure methods for handling challenging report formats.
My only immediate suggestion is that there is something about the models that is different in the way they understand what a 'page' is. In particular what indicates the beginning of a page.
What sort of input reports are you using? Text, PRN, PDF or something different?
As a generic starting point it might be worth checking the "Options" settings for each of the models, especially the Input options to ascertain if there are any differences. Other than that maybe take a copy of each model and modify the detail trap to pick up the whole of every line and then see which line is missing (or gained) between the models - that might give you a clue about where to look.
Beyond that I'm not sure that you can post anything for this particular problem on the forum. If you are in a position to share the report (i.e. it's not considered sensitive by your company) then I would be happy to take a look if you can make the a report and model examples available by, say, email or some other method.
I don't have a running version of 11.6 handy but do have 11.7.
Hope this help.