Seems to be related to the issue from this Githuib issue:
https://github.com/zurb/foundation-sites/issues/10604

I also struggled to get this to work, and gave up in the end.
I'm not familiar with Webpack and module bundling, so I'm sure that doesn't help, but trying to read the provided links and anything else I could come over, I can't for the life of me figure out how to set this up correctly.
The custom javascript I'm trying to include (as a custom.js living alongside app.js in the src folder) is not a "module" per se, it's stuff like initializing Slick sliders, and adding some custom event handlers to Foundation plugins like Reveal and Tabs.
Where (or how) would I best add code like this? Directly in app.js? (I tried, but it didn't seem to work) Make it into modules? If so, how? I would like to keep all bundled into one JS file on build to minimize requests.
I had to roll back to 6.3 in the meantime. Hopefully some easier guides will appear soon. I realise it may be my lack of skills, but from never having worked this way before, I've so far managed to learn NPM and Bower through the provided guides, but here I'm utterly lost.
I simply cannot understand how Webpack works, and what changes needs to be done to move from the recommended setup for 6.3 to 6.4. A step-by-step for relative dummies would be incredibly helpful. It seems I'm not the only one struggling with this.

I managed to find the solution myself – when the modal gets reset, I had to fire this to re-initialize the tabs plugin. Then it works again!
$('#film-info-tabs').foundation();