I notice that the latest release is an alpha version (4.00-alpha2) so perhaps the current maintainers released a version with basic functionality and plan to add to it later (although this is just speculation on my part).
While I try to help out with the Drupal Juicebox forum where I can, I am not a Drupal coding expert and don't have the time to dig into this beyond the reply post that I did on the Drupal forum. The "classic" version of Juicebox on Drupal, from when there was a full time maintainer there, relied on using the JuiceboxBuilder application to build your galleries and the associated XML file driving the gallery. That XML file for a specific gallery had a location for the caption field for each image in the format:
importCaption="IPTC"
enableAutoPlay="true"
autoPlayOnLoad="true"
captionPosition="BELOW_IMAGE"
captionHAlign="CENTER"
<< ... that's just a portion of the for a given gallery
I'm pretty sure that, after the "classic" version, an effort was made to let Drupal users build their own xml driver entirely within Drupal. I posted a partial solution of the Drupal forum showing that, under the right conditions you could still see the caption sources. I'm not sure that is a complete solution.
Within Drupal I'm working on converting a whole bunch (over 100 galleries and 10k images) to another solution using the Drupal Solo theme and some paragraphs bundles software. It's a work in progress. Since 2015 when I started with galleries on Drupal the core developers there have totally changed around the way media and media files on Drupal work and gallery software there can't ignore the changes. If you just upload a jpg file to the standard "files" directory on Drupal the newer software won't even recognize it ... it has to be converted to a so-called "media" file to work with the newer software. T'aint simple and I'll probably wind up dumping many of my images during the conversion.