Strange highlight when searching OCR'ed PDF

[softfoot]

Describe the bug
I often import PDFs and save them to get a searchable PDF, usually it works very well and when I search
for a phrase it highights it nicely.
However, with some PDFs instead of highlighting the phrase it shows a verical bar, see picture.
It is not the pdf reader (Foxit) as I see the same result if I open the file in Firefox and Chrome (unless they all use the same OCR engine!!)

To Reproduce
Steps to reproduce the behavior:

1. Import test PDF
2. Make sure OCR is enabled
3. Save as new PDF
4. Open new PDF in viewer
5. Search for "GPIB address"
6. View problem

Expected behavior
The phrase should be highlighted in the PDF viewer.

Desktop (please complete the following information):
Windows 7
NAPS Version 7.5.1.0

Additional context
If you require a PDF that shows this problem I can upload it, but it's big 3.2Mb