@Matt Baylis Welcome to Microsoft Q&A! Thanks for posting the question.
I understand that Azure function successfully gets a token, but Azure DevOps is not accepting it and getting 203 error.
Please refer the sample code for HTTPTrigger function. This sample shows how to get an Azure AD access token for a Managed Identity using Azure Identity client library for .NET and authenticate to Azure DevOps to create or get a work item.
Steps to call Azure Devops REST API using Azure AD access token are mentioned here.
Refer to this video tutorial which demonstrates how to connect to Azure DevOps using a User Assigned Managed Identity within an Azure Function.
Please note DevOps is currently not supported in the Q&A forums, the supported products are listed over here https://learn.microsoft.com/en-us/answers/products (more to be added later on).
You can ask the experts in the dedicated forum over here:
https://stackoverflow.com/questions/tagged/devops
https://developercommunity.visualstudio.com/spaces/21/index.html
**
Please 'Accept as answer' and ‘Upvote’ if it helped so that it can help others in the community looking for help on similar topics.