Dropdown "choices" field filled with Generic.List element when saving document

Reproduction steps:

1. Create new project
2. Create UI Document file (Project window > Create > UI Toolkit > UI Document)
3. Open UI Document
4. Drag in the "Dropdown" element
5. Check the "Choices" field
6. Save the document
7. Check the "Choices" field again

Expected result: The "Choices" field is not changed

Actual result: "Choices" field gets filled with "System.Collections.Generic.List`1[System.String]"

Note: Also reproducible when unsetting the "Choices" field and undoing the unset