Save for Web (Legacy) does not clip transparency to visible layers as it did previously
"Save for Web (Legacy)" , previously, when saving to PNG with transparency and NOT clipped to artboard, the transparency would clip to visible layers. Now it clips to existing layers, even when the layers are not visible.
Details previously posted in an Illustrator forum:
Mark Nicoll commented
This issue came in with version 24 and is still present in version 25. Here are some theads reporting this same issue with PNG exporting for you to vote for: