The bug
If you copy a lectern with a book on it in creative mode and keep its NBT data on the copy (CTRL+Pick block), the copy will have no book on it despite having the NBT for it.
Attachments
Comments

Likely related to the fact that the lectern has a "has_book
" state, though its book is saved within its NBT data and is not dropped when broken.

Still in 1.14 Pre-Release 3

Still affects 1.14 (Release), 1.14.1 Pre-1, and 1.14.1 Pre-2

Still affects the 1.14.1 release

Still affects the 1.14.1 release

It's in 1.14.4

Confirmed fixed in 1.16-pre5