Same problem in Mendix 7.9.0.
Workaround : just add a sort widget into your form and set your default sort in this object. you can hide it after.
tested under mdx 7.12.
This issue is fixed in 7.11
> Is this a bug in Mendix 7.3?
No, I tested it with Mendix 7.3 alone.
It is probably a migration bug for a specific database from a specific Mendix version.
If I were you, I would create the dropdown again because the system setting/configuration or somewhere hidden may be messed up.