I take a philosophical viewpoint with issues like these. Here’s the thing: does it matter if you get it wrong on WK? So, there’s basically two possibilities. Either, A. you won’t see it outside of WK in that time, get it wrong, it’ll get demoted, and you can review it again. Or B. you will see it in use outside of WK, remember you knew it, review it and now, since it’s in context it will stick.
With A, it’ll get demoted. And you’ll review. The “shock” of failing will signal to your brain that you need to retain the knowledge longer than a few days (it sounds like your problem is a classic short term vs long term memory problem) and that reinforcement will help you get it right the next time.
In any case, no harm done that it was locked away for a time, because you didn’t need it anyway.
B. is a little annoying in the “I used to know this!” sense, but still no big deal, because refamiliarizing yourself is always way easier than learning something for the first time. And because of that outside exposure, you’ll probably get it right when you see it in January. Yay.
“But,” I hear you cry, “I’ll get it right and I shouldn’t have!” People talk about this endlessly with SRS. “If I see information outside of the SRS, won’t I be polluting it?” Yes. Yes you will. In the same sense that painting a building might get paint on the scaffold. SRS is a tool for getting knowledge into your head. It isn’t a goal in itself. Once you start using the knowledge, your brain will naturally retain what it finds useful. And if you don’t use it, there’s no harm in letting it fall away. You don’t need it after all. So why work harder than you have to by doing extra reviews to keep it?
Something else you might not have considered. You might not know these kanji/vocabulary now, but you might in January. How, you say? WK is structured so you are constantly learning new kanji and vocabulary based upon previous material. There is a natural reinforcement going on. As you get a better sense of Japanese, kanji, readings, and so on, you build more structure in your brain. Some of those concepts which you’re having trouble recalling now will gain additional associations with kanji and vocabulary you haven’t even seen yet. Just encountering a new word with an old kanji helps every other word you’ve seen with that kanji become more firmly lodged. That structure will lift and support the knowledge that might be be difficult to recall now, bringing it to the surface when you actually need it. This is how our brains work. And this structure is your goal, not passing tests on WK.
So don’t get discouraged at this point. As others have pointed out, there are ways to review the material out of sequence, if it’s really bothering you. I know it can be anxiety inducing. Reducing that stress may be worth the extra effort for you. But I personally would let the SRS do it’s job a little longer.