Somtimes, text inserted by insert_text cannot be detected as a text layer by Abbyy, any reason?

[nissansz]

Somtimes, text inserted by insert_text cannot be detected as a text layer by Abbyy, any reason?

p31-1_350_textlines_ocr12.pdf

[nissansz]

I tried TextWriter, attached a result.
1st page text layer can be found by abbyy.
2nd page text layer cannot be found by abbyy.
Don't know the reason. I inserted text in the same way.
207.pdf