That is probably a bug. As far as I know, Tsurukame calculates the level up estimate based on the minimum possible time remaining, even if you usual fail reviews and will probably take longer.
Since it’s not an official wanikani app, and therefore not supported by Tofugu and the wanikani team, I’d make a post in this thread to let the developer of Tsurukame know.
Edit: as other people have noted, wanikani stats is a great place to find a level up estimate based on how long you actually usually take on levels.
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