Dragging and dropping the set will not drop the set in the location indicated by the transparent indicator. Instead the set will be placed at what seems a random position.

Statamic version: 3.0.42 Pro
PHP version: 7.4.9
Fresh install from statamic/statamic
Can confirm this issue, happens for any fieldtypes where it allows you to configure sets, like a replicator.
workaround that works for me atm, when adding new fields to fieldset:
add new field, save, reload, reorder, save :D