I see! That solved it!
However, I included the double braces because I have quite a few foreign language titles, and when generating the final document, it would transform the titles using titlecase. So Ceci est un livre would become Ceci Est Un Livre—which is incorrect. Is there another way to stop it from doing this? 

On Friday, July 30, 2021 at 11:23:22 AM UTC-5 John MacFarlane wrote:

Normally pandoc will try to extract a short-title from the
title field of a bibtex entry by splitting on the colon.
But this is being blocked in this case because you have
put the whole title in an extra set of braces.

Try with

title = {This is a title: with a subtitle},


MK <mende...-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> writes:

> You can see in the bibliographic entry that I posted (and below) that
> there's no short title field; however is there something in the CSL file
> itself that would generate short titles? Or is specifying a short title in
> the bibtex file necessary?
>
> @Book{somekey,
>
> author = {Doe, Jane},
>
> title = {{This is a title: with a subtitle}},
>
> publisher = {Yale University Press},
>
> address = {New Haven},
>
> year = {year},
>
>
> On Thursday, July 29, 2021 at 4:55:36 PM UTC-5 John MacFarlane wrote:
>
>>
>> Do you have a 'short-title' field in your CSL bibliography for this
>> entry?
>>
>> MK <mende...@gmail.com> writes:
>>
>> > Hi, I'm sure I must be doing something wrong, but I can't figure this
>> out.
>> > I'd like for the first instance of a reference to use the first title
>> and
>> > all subsequent instances to use a shortened title (whatever appears
>> before
>> > the colon). So for example the first instance would be:
>> >
>> > Jane Doe, *Some Book: Which is Really Good *(Place: Press, Year)
>> >
>> > While the second should be:
>> >
>> > Doe, *Some Book*
>> >
>> > However, each time it reprints the entire title, so even on the second
>> > instance it would be: Doe, *Some Book: Which is Really Good.*
>> >
>> > I'm using Chicago CSL and have tried both the one called 'full note,
>> with
>> > ibid.' and 'full note, short title subsequent.'
>> >
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>>
>
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