Are you sure the transcluded page itself is not protected? Are you sure you are not protected from editing or editing this partial page?

Cascade protection on plwiki:

https://pl.wikipedia.org/wiki/Specjalna:Zabezpieczone_strony?namespace=&type=edit&level=0&wpfilters%5B%5D=cascadeonly&size-mode=min&size=&uselang=en

When looking at transcluded pages of some of them I see the edit source link, but I have no idea where the orange icon from your image is coming from.

I cannot reproduce this. I am also interested in knowing where that colorful box comes from.

In T240532#5738180, @Ammarpad wrote:

I cannot reproduce this. I am also interested in knowing where that colorful box comes from.

The screenshot is from the interface in the Oasis skin, used in FANDOM wikis.

I would also be very surprised by this bug, because I’ve seen the exact opposite effect – I see “Edit” rather than “View source” on the enwiki page Wikipedia:Selected anniversaries/December 14, even though I don’t actually have the right to edit it.

As far as I understand, this is because MediaWiki’s “is probably editable” check (in PermissionManager terms, a “quick” rather than “full” or “secure” check), which among other things governs the “edit” / “view source” distinction, does not take cascade protection into account – it would be too expensive to do so on every page view.

Fandom uses a fork of a pretty old MediaWiki version (though they’re working on an upgrade), with hundreds of extensions. I suspect the bug is somewhere in there (who knows where exactly), not in current MediaWiki core.

In T240532#5739729, @Lucas_Werkmeister_WMDE wrote:

Fandom uses a fork of a pretty old MediaWiki version (though they’re working on an upgrade), with hundreds of extensions. I suspect the bug is somewhere in there (who knows where exactly), not in current MediaWiki core.

Yes, FANDOM uses an old version of MediaWiki that still has no {{!}} magic word, and an upgrade would cause so much traffic.

On my wiki, the main page is protected with cascading protection:

PokéSpołeczeństwo Wiki (the main page, protected [edit=sysop, indefinite] [move=sysop, indefinite] [cascade]
This also protects the following pages:

Template:Strona główna
Template:Strona główna/domyślna
Template:Wiki kolor/ciemny+
Template:Wiki kolor/ciemny
Template:Wiki kolor/jasny
Template:Wiki kolor/jasny+
Template:Pluralform
Template:Promień ramki
Template:Nagłówek

In T240532#5739729, @Lucas_Werkmeister_WMDE wrote:

I would also be very surprised by this bug, because I’ve seen the exact opposite effect – I see “Edit” rather than “View source” on the enwiki page Wikipedia:Selected anniversaries/December 14, even though I don’t actually have the right to edit it.

As far as I understand, this is because MediaWiki’s “is probably editable” check (in PermissionManager terms, a “quick” rather than “full” or “secure” check), which among other things governs the “edit” / “view source” distinction, does not take cascade protection into account – it would be too expensive to do so on every page view.

I believe I have seen this and later forgot where I saw it. It's a bug anyway, and should be fixed. Related: T219848