In our case, after we dynamically populate the ComboBox at runtime, we called SetSelectedOption to select the “None” option which should always be first in the list. After noticing some weird stuff in Unreal 5.3 with ComboBoxKey (e.g. SetSelectedOption broadcasts a field value change event before updating the value, which was added after UE 5.0 - but the ComboBox OnSelectionChanged callback handler will trigger a second event broadcast) I changed to using ComboBoxString and it seemed to fix our issue.
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