The DSpace 7 submission currently doesn't support a normal dropdown yet. This is a problem in both REST and Angular UI
The only way the DSpace 7 submission supports a dropdown, is via authority control. DSpace 6 supported both solutions, where the non-authority dropdown was the standard.
This implies e.g. dc.language.iso contains an authority ID (visible at manual submissions, e.g. https://dspace7.4science.cloud/server/#/server/api/core/items/05ff9d54-df92-43e8-899e-9a2f55438725, not submitted by me) if it is configured as a dropdown in DSpace 7.
This has a direct impact on the data being stored and DSpace currently treats authority values different from non-authority values on many parts (think about search, browse, CSV export, ...).
There are also quite a few bugs in the authority implementation of the dropdown, but these can be part of another JIRA ticket