It would be a good idea to share your computer's details (OS, graphics card, CPU, ram) and inDesign version.
I have the same issue, though it goes beyond just the gradient feather. On my computer it fails intermittently on any transparency effects (shadows, opacity, etc) and also doesn't load full resolution links on multiple pages, intermittently loses the anti-alias of text, and sometimes loses whole chunks of the page and just shows white boxes until zoom level is changed.
inDesign CC 2015.2
Feature was working better in previous versions of inDesign / OSX. I often have to turn off Overprint Preview to get an "accurate" view of the document, which is extremely disappointing when designing for print.
iMac 27-inch, Mid 2011
OS X Yosemite. 10.10.5
3.4 GHz Intel i7
16 GB 1333 MHz DDR3
AMD Radeon HD 6970M 2048 MB
Posted this before in another thread. Also tried to start a discussion but there's no trace of it anywhere, so it must not have been posted.
I have access to 4 other computers and I will try to test on those since I suspect this is a CPU / GPU issue.
Thanks for your reply! I am having ALLl of the other issues you describe as well (except for the chunks of page and white boxes).
I’ve attached my Mac info (very different than yours), but I’m using the same InDesign version you are.