to give a brief description,
1. the Modern library has 114 document sets at the root level
2. Inside the 114 document sets are 168,831 Items (lots of folders)
3. none of the decoded paths I checked exceeded 400 characters, but I only checked a few
4. The library does not inherit permissions
5. No folders are exceeding the list view threshold
6. As far as my testing, this only happens on a fresh page and the first time you drill into the folder, drill into it a second time and no issue, refresh the page and the problem comes back.
7. Happens in Edge and Chrome
When drilling into random folders/Document sets users actually get bumped back to the root folder, sometimes from the first subfolder and other times, deeper into the folder structure. The users are reporting that this does not happen in Classic view and I confirmed as much.
Never seen this before, thanks for any help