This was mentioned in an earlier thread. It appears that converted layouts which have not had any theme applied will print as expected. However, new layouts pick up default settings using theme CSS, which may be controlling some of the settings for specific window modes so that print and preview are not always consistent.
This might also be attempted with conditional formatting. You could try adding conditional formatting based on Get (WindowMode) > 1 to see if you can get print and preview to match. I don't know if that will overwrite whatever CSS may be doing in the background.
(Preview mode = 2, printing in progress = 3; they are separate modes in the latest few versions.)
And complain directly to FMI that print and preview really should match in their themes. I can't think of any reason these window modes should diverge.
Thanks for your answer.
I actually just found what the issue was, and for once, the new theme system is not to blame. After adding a theme to the converted layout, I noticed that it was still showing the lines in preview mode. So I started to look everywhere for an option I could have missed and actually theres a new check box in the layout setup which is "Delineate fields on current record only". A converted layout will have it unchecked, but a new layout won't.
After unchecking this box, all my field borders now appear again in preview mode.
Interesting, and thank you for the report on your results. It's important to know what really resolves an issue.
Most interesting! Thanks for passing that along.