Steps to reproduce:
1) Open the attached project.
2) Open scene 'BugReportScene'.
3) Enter play mode.

There are 2 animated .fbx models in the scene - the one on the left has the 'Optimize Game Objects' option selected in the Import Settings, and the right one does not. The first object is animated incorrectly - the animation is blinking and glitchy, some parts are flipped. Unchecking the optimisation option makes both models behave the same.

Reproduced on:
5.5.0a1, 5.4.0b21, 5.3.5p3, 5.3.4p4.

Did not reproduce:
5.3.4f1, 5.2.4f1