I also have been encountering issues for the pop-ups in the Map Viewer.
These pop-ups are configured for an image service.
When zoomed out to the overview level of the service, the pop-ups display reliably, as expected. When you zoom in to the level of the source rasters, the pop-ups almost never display. The "thinking wheel" shows briefly, but then nothing displays. Sometimes, if you hit just the right spot, it will magically work, but most of the time it doesn't.
Based on some of the comments in this thread, I opened the map in Web Viewer Classic, and the pop-ups work fine at all zoom levels.
Are there zoom-level pop-up visibility settings in the new Web Viewer that I'm not seeing?
@RussRoberts This map is not ready to be shared publicly, but I could invite you to a group with access to the map if that would help.