I don't know (as this does not happen for me).

Maybe a small chance for an idea: CSTAL2 NNUE download package was provided with a polyglot-books directory included. Two books inside: a large one, book.bin, and small one Rebel.bin

I think I remember I failed to enable these books if using the CSTAL2 exe (maybe a specific issue to Arena?)
The work around I found: use polyglot.exe as the front-end.
Of course, polyglot has no issue using bin books.
Polyglot uses a .ini file linking to both book and back-end engine (here CSTAL2 NNUE). But you may also include many other statements inside this .ini; isn't Korniloff stated here? Copy-pasting both polyglot.exe and a sample .ini is quite usual, to avoid reinventing the wheel. Easy to leave unwanted statements inside the .ini... Worth a check.