Subtitle if "alternative dividing punctuation mark" is only on spine

The style guide mentions that colons should be used to separate subtitles, except if there is an “alternative dividing punctuation” mark, e.g. an en-dash.

Does this apply only if said “alternative dividing punctation mark” is used on the front cover?
I have a couple of releases where the front cover does not have such a punctuation mark but the spine does (usually an em-dash or an interpunct).

Personally, I’d use what’s on the spine then but what do the experts say?

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The front cover is often beset by artistic flourishes, while the spine is typically more typographically standard. I’d go with the spine, too.

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