As others have said, I think this is fairly natural. After I hit 60, I started progressively un-burning kanji in each level. The regressed items seem to have fallen into several categories:
Fairly uncommon kanji that I’ve just outright forgotten. Lots of my material such as graded readers don’t use a wider variety of kanji, so it is a problem that even with reading practice I just don’t encounter some. Lots of material will include up through 6th grade kanji, which is a pool of kanji that doesn’t map well to WK’s levels.
Kanji that have either similar meanings or similar appearance that are easy to mix up. I think that depending on how long it has taken to go through all levels (3+ years for me), it is possible that these aren’t at play at the same time, so the particular kanji stunt double has to be re-learned.
Kanji that are very difficult to remember on their own, but easy to recall in vocab. I’ve had a shamefully low retention rate in some levels in the teens and twenties. (I’m up through about level 26 in resurrecting - I try to do a level a week.) However, if I see the vocab, I very often just know what it is. In some cases #2’s kanji often are easy to mix up on their own, but since they have the same reading, the vocab is easy to retain. I also find I learn the items much quicker on the second go around.
Also, I’ve found that when reading at a natural pace that some words pop into mind very quickly, but in my un-burning process, were not easily retained out of context. Therefore, my advice is to practice reading more at lower levels in spite of encountering a lot of unknown kanji along the way. Read online with an extension to help there, and/or get a Kindle Paperwhite with several dictionaries to help. My biggest frustration here is comprehension of what is being said - it is easy to get wrapped around the axle and focus very much on reading, but not retain what is being said well enough to make it enjoyable.
It is a really tricky balance of speed of WK, study of grammar, reading practice, and speaking practice. I think finding the right ratio that works for the individual is something to strive for - always re-evaluate what you are doing. In conclusion, don’t sweat it too much, even Japanese people regress on kanji!