[@jj9617 Participant 8 years, 10 months ago]

I found this topic from 3 years ago, which helped me:

Pagination broken when using shortcode for topic index

It suggested changing the Forum setting “Forum root should show” to “Topics by Freshness.” I had it set to “Forum Index.” With this setting changed, my pagination works.

Not sure why this setting would break pagination in bbpress topics index shortcode, but at least it works now.