Problems with paragraphs widget when paragraph contains media widget

Problem/Motivation

I'm having an issue on one single site, in a multisite installation. All sites in the installation have some common features, including the one in which this problem is appearing:

- A custom paragraph type, "Image," which includes a media reference field and a URL field. The media reference field uses the Media Library widget.
- A custom block type, "Slider," which includes a paragraphs field that permits the Image paragraph type. The paragraphs field uses the Stable widget, although I've encountered the same problem with the older legacy widget as well.

The problem: when a user adds a new paragraph, they are presented with the media library widget as expected. Clicking "Add media" opens the media library modal. Selecting or adding a media item works as expected. Upon hitting "Insert media," the modal closes and the Paragraphs widget does not update to show selected media. Saving the entity results in the new slide being added, but the media reference has not been made.

One variation: if the widget isn't set up to have paragraphs collapsed by default, then upon loading the form, the paragraph subform is *not* visible within a new paragraph.

The "Inline entity form - complex" widget works properly.

No javascript errors appear in the browser console. No PHP errors appear in the log. It feels like an ajax error, but no errors support that.

Testing some other entity types, and testing some new ones I set up for the purpose, I was able to determine that it happens in *any* paragraphs field that contains *any* media library widget, regardless of the media type(s) allowed for selection.

Media reference fields using the media library widget work fine, as long as they are not nested in a paragraph.

No other site in the multisite installation encounters has this problem, despite sharing the configurations for these entities and forms.

It happens even with administrative privileges.

I've uninstalled any modules that weren't in use on other sites in the multisite installation...and quite a few that were.

Obviously, since I can't even duplicate the problem on other sites in the same installation with the same configuration, I can't give good guidance on duplicating the problem. At this point I'm just casting about wildly in the hopes that someone may have seen similar behavior at some point, and can suggest a fix, or at least an avenue of investigation I haven't tried.