The action "Download Definition" (aka ExportYamlAction) doesn't always handle YAML Syntax Characters correctly. In the case where you had a JCR based definition where one of the property values starts with one of these characters it can create an invalid definition.

Reproduce
Create an app definition in JCR where one of the configuration property values starts with one of the reserved characters.

Expected
The value should be surround by quotes so that the YAML parser does not mistake it for some sort of YAML specific functionality.

Actual
The value is left as-is which can lead to problems with the definition validity.

Workaround
After downloading the definition add the quotes manually in the editor.

Development Notes
ExportYamlAction is already deprecated since 6.2.