How to reproduce:
1. Open the attached project
2. Open main scene
3. Open Animator Window
4. Click Punch there
5. Check Mirror to play to see the animation
6. Uncheck Mirror to play to see the difference

Expected result: Both animations are in the same position doesn't matter if mirroring is enabled or not
Actual result: Animation which has mirroring enabled is 0.5 cycle offset

Reproducible: 5.2.5f1, 5.3.6p6, 5.4.1p2, 5.5.0b5

Note: tested some ninja warrior animations from asset store and this issue didn't occur

By design: This helps users setup loops/transition between locomotion clips.