My book has finally passed! Several hours to debug...
I made the following observations:
1) I was working with book mock-ups that I made 4 years ago => Maybe this is a problem with the successive versions of LR since.
2) On my problems, the Myriad Pro font was problematic: The book editing interface under LR is very poorly done and bugged, I spent hours removing text fields with this f.... font, but even if the slightest text field, but invisible and hidden, was in myriad police => error! And the fact of resetting all text fields, or assigning a classic font to all text fields => There were still text fields with this f.... Myriad font
That’s why I say that this module under LR is bugged!
3) Once again, LR’s book module is bugged because even if you format the page numbers, it may be that randomly, the font size varies, and the font also (still f..... Myriad font)
4) Test on a new book with one page, Myriad font ... it works!!! hence my conclusion on the old book layouts poorly supported by the new versions of LR
GENERAL CONCLUSIONS
It is unacceptable for an Adobe software, Lightroom in this case, to have this kind of bugs.
Please modify your text module in the books module.
Clarify the options: apply a default style to all page caption fields, or all photo caption fields, that work.