Quick answer: Construct 3 collision polygon stays in original orientation when sprite is mirrored? Polygons are sprite-frame relative - re-define for each mirror state or use bounding box.
Enemy walks left, mirror-flips the sprite. Collision polygon stays oriented for right-facing.
Use bounding box collision
For symmetric enemies, default bounding box adapts to mirror. Polygons are for asymmetric shapes.
Or duplicate frames per direction
Author left-facing and right-facing as separate frames. Each has its own polygon.
Verify in the polygon editor
Image editor > Set collision polygon. The preview shows the polygon over the unflipped image; mirror at runtime can violate the design.
“Collision polygons are frame-locked. Mirror is a render transform; polygon doesn't follow.”
For platformers with asymmetric enemies, dual-frame design is standard. The extra storage is worth the collision correctness.