Pull request pipeline does not run if another PR is merged into source branch

Given a pipeline is configured for pull requests.

When the user opens a PR A from a given source branch A to a given target branch B.

The pipeline should run correctly for PR A.

When the user pushes a new commit into the source branch A, if the PR A is not yet merged or closed.

The pipeline should run again for PR A.

But when the user opens a new PR B from the source branch C to target branch A and PR B is merged.

The pipeline will not run for PR A, even if the head of source branch A has been updated.