The issue could be generalized to the "note" field in general. I use it, for example, for online documents:
note={\url{https://....}}

IIRC the LaTeX template is set to use biblatex, so adding support for ISBNs might be as simple as passing the isbn=true parameter to \usepackage{biblatex}.

Thanks for the suggestion @logological. I just tried that but it didn't seem to affect the output. I suspect it might be something to do with our CSL.

@tarleb - do you have any suggestions?

I agree that this looks like a CSL issue. ISBN is a supported CSL field, but apa.csl does not seem to support it. As far as I can tell from a quick search, ISBNs are used in quite a few cases. Including it should be easy following, for example, iso690-numeric-en.
Of course, the resulting style would no longer be stock APA.

One of the authors of biblatex has indicated how to get the APA style to include ISBNs. He also warns that with this modification, the style is no longer fully APA-compliant.

Of course, the resulting style would no longer be stock APA.

Yeah, I'm not sure we care too much to be honest. I personally would rather we showed ISBNs when we have them than be APA-compliant.

Thoughts @openjournals/joss-editors?

I agree - content beats style