Mac OS X 10.13.6
Acrobat Pro DC – Version 2021.001.20138

Inserting a text watermark using the Helvetica Neue font enters a series of boxes instead of the text. All weights and slopes affected except, bizarrely, the ‘Thin’ weight.

Inserting text using the Helvetica Font produces the error message “The font ‘Helvetica,Xxxx’ contains an invalid encoding. Some characters may not display correctly”. This error message appears on saving the PDF and every time it is opened in Acrobat Reader DC.

Both fonts are system fonts and work satisfactorily in all applications (e.g. InDesign, Word) including Acrobat Pro DC, but not as a watermark. The fonts check-out as undamaged, and are not in conflict with any other fonts.

This bug is duplicated on more than one installation of Acrobat Pro DC on separate Macs.

It looks suspiciously like a bug.