Amplify diff deploy: how to only redeploy on changes to a part of a monorepo?

We have a project in a monorepo: the two folders at the root of our project are named api and ui. We use Amplify to deploy and serve the front end (ui) but not the back end (api). We only want Amplify to redeploy on a change in the ui folder. We tried setting the following environment variables in the AWS Console at AWS Amplify -> our app -> App settings -> Environment variables:

AMPLIFY_DIFF_DEPLOY=true

AMPLIFY_DIFF_DEPLOY_ROOT=ui

But this did not work: a change pushed with only changes in the api folder triggered a build and redeploy. We tried with

AMPLIFY_DIFF_DEPLOY=true

and

appRoot: ui in Build settings

But again this did not work: a change pushed with only changes in the `api folder triggered a build and redeploy. Finally, we tried

AMPLIFY_DIFF_DEPLOY=true

AMPLIFY_DIFF_DEPLOY_ROOT=ui

and

appRoot: ui in Build settings

alas, to no avail: still got a build and redeploy and a change in the api folder (no change in the ui folder). How do folks get Amplify diff deploy working?