I wonder if that may be the source of the -line pronunciation. The book must have made it clear how to pronounce it, as you wrote. So maybe some people thought this was the classic way or something. In the comment section for
Madeline lots of people insist on -line and that -linn is only correct for
Madelyn (which is a bit odd because
Madelyn is just a variant spelling as far as I know like Michaela/Mikayla etc.).