Sorry 😞
Edit:
Ok, I uploaded them as attachments and referred to them in my comments.
It's even worse than I thought.
It also happens when opening doors (at least I assume it's the same bug):
(see screenshot 2015-09-25_20.30.18.png)
I have the same bug on Mac OS X 10.10.5 (Java 8 update 45) playing on a server running on Mac OS X 10.6.8 (Java 6 of course), and the farmland also does not graphically change.
(see screenshot 2015-09-25_19.54.30.png)
It carries on even when you plant seeds in on adjacent block (as shown in submitter's image) but clears up after a short while (presumably because of random ticks) or if you place a block next to the seeds. (presumably a block update)
I can confirm that, as of 15w45a, this bug has been resolved. I tested it exactly as it occurred originally, but with the new snapshot, and it didn't happen.