Couldn't update `branch` property of a `sourcecontrols/web` resource

`branch` property of a `sourcecontrols/web` (name: `web`, type: `Microsoft.Web/sites/sourcecontrols`) resource couldn't be updated from https://resources.azure.com/.

When I execute a PUT request to https://management.azure.com/[app-service-id]/sourcecontrols/web?api-version=2018-02-01 operation succeeds, but value of the `branch` property remains the same.

Start by reviewing the PUT endpoint for Microsoft.Web/sites/sourcecontrols/web with api-version=2018-02-01 and the reported behavior from resources.azure.com. Confirm whether the API specification reflects the observed branch-update behavior; done means the discrepancy is explained and the relevant issue or specification has an appropriate resolution.

Written by the indexing model from the issue text.