assign_coords reset all dimension coords to default (pandas) index

[benbovy]

What happened?

See https://github.com/martinfleis/xvec/issues/13#issue-1472023524

What did you expect to happen?

assign_coords() should preserve the index of coordinates that are not updated or not part of a dropped multi-coordinate index.

Minimal Complete Verifiable Example

See https://github.com/martinfleis/xvec/issues/13#issue-1472023524

MVCE confirmation

• Minimal example — the example is as focused as reasonably possible to demonstrate the underlying issue in xarray.
• Complete example — the example is self-contained, including all data and the text of any traceback.
• Verifiable example — the example copy & pastes into an IPython prompt or Binder notebook, returning the result.
• New issue — a search of GitHub Issues suggests this is not a duplicate.

Relevant log output

No response

Anything else we need to know?

No response

Environment

Xarray version 2022.11.0