With the below repro steps, the Humanoid pose in the Scene (in this case T-Stance) is changed after previewing an animation in the Animation Window.

repro:
-open project
-open loco scene
-select Winston GO
-open Animator window and move a node/state in the graph (ex: Blend Tree node).
-without un-selecting Winston, open the Animation Window and scrub the timeline to preview the clip animation.
-now disable the Animation Window Preview button...
bug: character is no longer in a T-Stance in the Scene
expected: character should snap back to the scene pose (T-Stance, in this case)

note1: this bug is not repro if you do not first move a state in the Animator.
note2: this is REALLY bad because it permanently changes the default scene and cannot be un-done.... scene corruption.
note3: this is not a regression since it's been there since at least 5.6