Its because you include the basepage in the subpage.
{{Album Release Details Header}} includes {{msgnw:{{BASEPAGENAME}}}} into a variable, which has the gallery (apperently the nowikiness does not apply to <gallery>. that might be a bug). Thus in a sense the image is really on that page.

Oh you are exactly right, Bawolff. Thank you for taking the time to investigate it in-depth like you did. I've done some further testing on my end, and you are correct that it's a problem with the {{msgnw}}.

Interestingly, if i have two images on a page (one in a gallery / one not in a gallery), it will show the one in the gallery as being linked. However, the image that isn't in the gallery does not show up in the "whatlinkshere" page (and this is the proper behavior).

So, it looks like you're right that the "nowikiness" isn't being applied to the gallery tag, but it needs to be.

Thanks again for digging.

Needs adjust of header or making INVALID?

Possibly a wontfix. msgnw is really not used for anything afaik, and mostly kept for back compat. Do we really want to mess with that back compat?

Basic issue appears to be that msgnw uses PPFrame::RECOVER_ORIG, which doesn't recover xml-style extension tags.

Change 123516 had a related patch set uploaded by Jackmcbarn:
Make RECOVER_ORIG preserve extension tags

https://gerrit.wikimedia.org/r/123516

(In reply to Bawolff (Brian Wolff) from comment #4)

Possibly a wontfix. msgnw is really not used for anything afaik, and mostly
kept for back compat. Do we really want to mess with that back compat?

For the record:
[01:22] <bawolff> present me thinks the backwards compatibility concerns that past me had were overrated

cscott-free: oh wow, that's black magic
***cscott-free makes the sign of the cross
cscott-free: i believe in fixing old brokenness
cscott-free: parsoid doesn't implement <gallery>, RECOVER_ORIG, or msgnw currently -- so making the thing that we eventually have to implement more sane is a plus
cscott-free: (parsoid currently uses the PHP preprocessor for many things, including extensions and magic words)

Change 123516 merged by jenkins-bot:
Make RECOVER_ORIG preserve extension tags

https://gerrit.wikimedia.org/r/123516