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.
I thought I was also running into this recently. But, I was wrong. Subitem duplication, when duplicating the parent, is handled asynchronously. That is, it can sometime take a little longer than expected.
However, if it continues to be an issue for you, I would recommend contacting monday support at: support@monday.com