We are facing exactly the same issue. In our case it’s just for one branch (other’s build just fine) that keeps on gathering pending jobs on it’s queue. Apparently the commit that caused this problem failed it’s build, but the Docker Hub didn’t manage to change the jobs status. Consequent pending jobs can be cancelled, but the problematic one shows for a second it’s cancelling and then it’s right back to untouched Cancel button. It also doesn’t show it’s commit ID or tag on the Builds tab. Tried launching the build job on that branch manually, and it eventually reported a FAILED status, though any new commits still appear as pending.
As a hotfix we started making the builds for that branch locally, tagging them, and pushing them to Docker Hub. (That still launches appropriate webhooks for consequent CD.)