Problems with duplicating subitems

When I try to duplicate an item that has subitems, the the subitems appear to get stuck. See the screenshot below; under the duplicated item I see “Subitem” greyed out and the little bouncing ball… which doesn’t stop bouncing. This is using the current version of Chrome (Version 85.0.4183.102).

Weirdly, when I tried it in Microsoft Edge (barf), it worked fine - though with one additional problem: it created an extra Subitem so that instead of the 7 I was trying to duplicate, I now have 8.