To reproduce:

• Checkout a VFC
• Click the 'Add Property' button
• In the 'Add Property' modal enter details for the property including 'Default Value'
• The property and the 'Default Value' appears to be added correctly, but when open same property for editing it can be seen that the 'Default Value' is not present

Exporting the generated TOSCA it can be seen that the 'Default Value' is present as expected.

Workaround:
Create the property first then add the 'Default Value' in the 'Update Property' modal