The trigger needed to be present in the branch which was configured to be the default branch for the pipeline, as this is the branch that is used to consider PR Triggers. For me this was the develop branch.

"If your pipeline completion triggers don't seem to be firing, check the value of the Default branch for manual and scheduled builds setting for the triggered pipeline. The branch filters in that branch's version of the pipeline are used to determine whether the pipeline completion trigger initiates a run of the pipeline. By default, Default branch for manual and scheduled builds is set to the default branch of the repository, but you can change it after the pipeline is created."