Calling Scene.frame_set() in a script, even if it doesn't change the frame, breaks calls to bpy.ops.pose.paste() and bpy.ops.poselib.browse_interactive(). They report success, but the pose is not changed.

Curiously, it doesn't matter when you set the frame. Before copying, before pasting, or after pasting; it always breaks.

Example:

import bpy

bpy.ops.pose.copy()

bpy.ops.pose.rot_clear()

bpy.context.scene.frame_set(1) # comment this line and paste() works

bpy.ops.pose.paste()