How to show recent builds for a tag, in APIv1.1

In API v1.1, I can list recent builds on a project at:
https://circleci.mheducation.com/api/v1.1/project/github/USER/PROJECT

I can also list all recent builds for a particular branch like this:
https://circleci.mheducation.com/api/v1.1/project/github/USER/PROJECT/tree/BRANCH%2FNAME

We have a project that builds whenever a new git tag is pushed to the corresponding github project. These builds have no branch name. They show up in the CircleCI web UI with the tag name where the branch name would be. When I list these jobs using the API, they show the value of branch as “null”.

I can’t seem to find any way to list these builds, unless they happen to be the most recent builds for the project, in which case I can just list all recent builds for the project (as in my first URL above). But if the most tag builds I’m looking for aren’t recent and there are many master or branch builds more recently, then the tag builds don’t show up at all.

I’ve tried things like this, but they always return an empty list:
https://circleci.mheducation.com/api/v1.1/project/github/USER/PROJECT/tree/null
https://circleci.mheducation.com/api/v1.1/project/github/USER/PROJECT/tree/TAGNAME

I see a post in the forum suggesting that the latter used to work, but stopped working several years ago, and unfortunately nobody from CircleCI responded:

How can I list the most recent builds (or all builds) for a particular tag? Or even, show the most recent builds for all tags, or all builds for branch “null”?