Onyx Boox M96 plus: Export issue

Hi,

I am an academic, I mostly used the M96 to read pdf, highlight them, and then export it to my library on my PC.

The thing is, on some pdf, scribbles and highlight are not exported correctly.
The line are on the pdf, but the text is no longer there.

I got the last firmware, and the latest stable android version 4.0.4

Any clue ?