Edited: Never mind.
Found a discussion about it here https://github.com/SharePoint/sp-dev-docs/issues/3947
Reorder of FieldLinks with CSOM no longer works. https://docs.microsoft.com/en-us/previous-versions/office/sharepoint-server/ms414768(v%3Doffice.15)
We get the following exception.
Value cannot be null.Parameter name: s
This started a few days ago.
Anyone else seeing this?