Bug? My character bone flips 180 degree without reason

Hello! I’m wondering why my character bone flips 180 degree without any reason.

char_frog_small_bug

I created this character from 3DS Max.
I already made lots of character in 3DS Max, none of them has the same issue.

Here is my unsuccessful attempt :

  • change FBX version from 2014 to 2018
  • change FBX format from binary to ascii
  • change rotation type from Euler to linear/TCB
  • add more rotation keys
  • change FBX axis from Y to Z.
  • change animation key tangent from smooth to linear.