Should one italicise an italicised phase within a book title?

Practice varies:

There are a couple of books with Forum Non Conveniens in the title: Forum Non Conveniens: History, Global Practice, and Future Under the Hague Conventions on Chioce of Court Agreement By Ronald A. Brand, Scott R. Jablonski

Forum (Non) Conveniens in England: Past, Present, and Future By Ardavan Arzandeh

and a chapter title from Private Law in the International Arena: From National Conflict ..., Volume 1000 By Jurgen Basedow, Kurt Siehr

Any will do: none look strange.

A UK style guide's advice

The Oxford Guide to Style (2002) addresses this problem as follows:

Occasionally it may be necessary to indicate italics in text that is already italicized, especially in titles or foreign text. In this instance the opposite font—roman type—is chosen:

[Relevant example:] La Physiognomie arabe et le Kitâb al-Firâsa de Fakhr al-Dîn al Râzî

It may look odd in running text to have a title end in roman type followed immediately by the resumption of roman text:

I've just finished A Study of Dickens's Hard Times. The conclusion was excellent.

As an alternative, some publishers set italics in italicized text by underlining it (A Study of Dickens's H̲a̲r̲d̲ T̲i̲m̲e̲s̲) or putting it within quotation marks (A Study of Dickens's 'Hard Times').

Later, in a discussion of how to handle titles in references, OGS again addresses the problem of it calls "titles within titles":

Book titles within titles are printed in the opposite font, that is, not italicized:

Lewis White Beck, A Commentary on Kant's Critique of Practical Reason (Chicago, 1960).

Antonia Tossoni Benvenuti, L' Orfeo del Poliziano (Padua: Editrice Antenore, 1986).

One alternative is to place the book title within quotation marks. This works better when the book title begins or ends the article title, as in the first example above. ...

Another alternative—to underline the matter—is possible, though seldom the best option:

Lewis White Beck, A Commentary on Kant's C̲r̲i̲t̲i̲q̲u̲e̲ o̲f̲ P̲r̲a̲c̲t̲i̲c̲a̲l̲ R̲e̲a̲s̲o̲n̲, (Chicago, 1960).

To judge from OGS's treatment of the topic, the usual treatment for titles within titles (which I believe would also apply to foreign words within titles) is to use "the opposite font" for that portion of the title: italics if the title is otherwise set in roman type, and roman if the title is otherwise in italics. The chief alternative treatment, OGS suggests, is to put the embedded title in quotation marks—as, for example,

Lewis White Beck, A Commentary on Kant's 'Critique of Practical Reason' (Chicago, 1960).

And the third—but "seldom the best" option is to underline the embedded wording. It's hard to imagine a case where this would actually be the best choice, but perhaps a situation where the embedded title had quotation marks within it would be a good candidate.

The example that appears in the posted question—

Forum (Non) Conveniens in England: Past, Present, and Future (Hart 2018)

falls squarely in to the specific subcategory of titles where OGS feels that quotation marks are most justified—namely, "when the book title [read 'foreign phrase'] begins or ends the article title." In that case, Oxford would seem to approve the following form:

'Forum (Non) Conveniens' in England: Past, Present, and Future (Hart 2018)

But OGS also seems fine with the first option, despite the result's being, as the OP notes, "awkward looking":

Forum (Non) Conveniens in England: Past, Present, and Future (Hart 2018)

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A U.S. style guide's advice

With regard to U.S. style preferences on this point, The Chicago Manual of Style, sixteenth edition (2010) takes a less flexible—and yet less explicable—line than The Oxford Guide to Style does:

8.171 Italicized terms and titles within titles. Any term within an italicized title that would itself be italicized in running text—such as a foreign word, a genus name, or the name of a ship—should be set in roman type (reverse italics). A title of a work within a work, however, should remain in italics and be enclosed in quotation marks.

From Tyrannosaurus rex to King Kong: Large Creatures in Fact and Fiction

The Big E: The Story of the USS Enterprise

A Key to Whitehead's "Process and Reality"

CMS offers no explanation of why it recommends handling titles within titles differently from foreign words within titles.