Pay close attention to the dates you're submitting:
Here is 26 Elul 5777 as an annotated screenshot:
What it has at the top of the page is correct. It's what's in the box next to the "convert" button that's bugged.
Notice that this is a Sunday, and that 16 September 2017 = 25 Elul 5777, a Shabbos, behaves normally. Compare other years and you'll see this trend hold up, your example in the question following this pattern as well.
What I think is going on here is that the software sees that Rosh Hashanah is during that week, so it says that "the week beginning 17 September is the first week of 5778, so I'll output 5778 into this box down here." At the top of the page, where the calculation occurs, is the important part, and at least that calculates correctly.
You might notice that the opposite doesn't happen; 23 Teves 5779 was indeed 31 December 2018. For whatever reason, this bug only occurs on the Hebrew calendar, not the Gregorian one.
I'll leave you with the honors of submitting the bug report.