RED rectangle instead of barcode on Android Adobe Acrobat

When I open some PDF on my Android (OnePlus 7, Oxygen OS 10.0.1) using the Adobe Acrobat app I see a red rectangle instead of the barcode.

I read similar problems on the forum, but always you ask for desired file instead of explain where is the problem, and how WE, THE COMMUNITY, can solve it. 

WHY?

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With regards

Tom