Can't update traffic splitting from AZ CLI

**Describe the bug**

I'm trying to update my traffic splitting distribution with `az resource update`, in vain.

**To Reproduce**
Steps to reproduce the behavior:
1. Try to update traffic splitting with a command such as `az resource update -n XXX -g YYY --resource-type Microsoft.Web/staticSites --set 'properties.trafficSplitting={"environmentDistribution": {"env":100}}'`
2. No update or error

**Expected behavior**
It should at least errors out I guess, but I'd prefer if it'd work.

**Additional context**
I'm wondering if it's not due to this property not existing in the latest API version, even though the resource does have it when querying it from the portal or the CLI.