Iframes run snippets always in their top window

Environment

Both latest and current stable ABP are affected.

How to reproduce

1. Go to https://webreflection.github.io/eyeo/frames/
2. Add webreflection.github.io#$#hide-if-contains hidden span filter
3. Optionally add webreflection.github.io#$#log ok filter too
4. Refresh the page to see filters in action

Observed behavior

The <span>hidden</span> element is removed only from the main page, but not within its iframes.

Each snippet is run 3 times (see 3 ok logs count) yet targeting always the same main page/window/document.

Expected behavior

Each iframe should run snippets within its own window/document, so that there should be still 3 ok logs, but each document should also not show the <span>hidden</span> element in its own body.