Acrobat Reader DC problem with PDF signatures

Hi! Our company ran into a problem with Adobe Acrobat Reader DC (in the following: Reader). If we digitally sign a fillable PDF document with our software, Reader validates it successfully. But then if we create a second signature with Reader, our first signature becomes invalid and Reader tells us that the signed content changed - although our first signed revision does not have one bit change! We would like to know what does the signature policy of Adobe contain for this specific case - how can a valid signature become invalid with modifications only AFTER the signed revision? We only ran into this problem with a few fillable PDF documents, otherwise everything works as it should be.