I'm not sure what you mean by "top panel just disappeared." A screen shot may be helpful for that reference.

Meshmixer determined four flaws on the head and repaired them. Simplify3D also identified problems, but I don't use S3D for repairs. The Meshmixer repaired model showed only self-intersecting surfaces, which may be a hint to the overall problem.

PrusaSlicer has determined that there's a problem about the 6.5 mm level. It appears that the wall thickness is too small. The model is smaller than 8 x 8 x 6 mm and that may contribute to the trouble. When scaled up by 10x, the problems disappear.

If it is not your creation, it's possible that the artist responsible overlooked a few important characteristics for 3D printing.