Is the WK system flawed?

I have the same issue with forgetting burned items. Fortunately there are indeed a few ways to review them.

You can reset individual items to Apprentice 1 by going to the items’ page and clicking the “Resurrect” button at the bottom. Then they’ll appear in your review queue as normal.

You can also use this Self-Study script: [Userscript] Self-Study Quiz. With this, you can quiz yourself as much as you want with customized quizzes. You can study burned items, leeches, everything you’ve ever learned, etc.

WK has plenty of issues, many of which are fortunately somewhat alleviated by userscripts.