Transparency gradient mask
I use Illustrator CC 21.1.0 on Mac OS X El Capitan ver 10.11.6
I encounter a problem with transparency opacity mask. Basically it doesn't cover properly the area where it should be fully transparent (black part). Despite the colour being set to 100% black ant opacity 100% there still is a hint/shadow of colour. In this case I would expect that from the bottom gradient goes up gradually form completely transparent to 100% opaque. And this going on for some time. See attached screen shot.
Please, tell more about your Preview mode, document color mode and 'Appearance of Black' preferences.
Also try to juggle them and see if anything changes.