Hi Mahesh,
This is usually related to the widget being hidden initially.
When the radio button changes to Yes, the dropdown/searchable selector becomes visible, but the widget may not be fully initialized yet. As a result, the first selection doesn't trigger the On Change event.
A few things you can try:
Could you share which Searchable Selector widget you're using (Marketplace link/version) and whether the dropdown is shown using Conditional Visibility or inside a snippet?
Kindly mark this as the accepted answer if it helps.