When pull-request updated by merging branch, pull-request pipeline does not run again.

Context:
Repo has a pipeline with a pull-request "release/**" step.

What I expect to happen:
If a pull-request on said branch to master is open and the pipeline steps have previously ran successfully (for a pull-request of the "release" branch), the pipeline should re-run the pull-request "release" steps if the branch has been updated by a pull-request merge from a "feature", etc. branch.

Right now, the only way to do this is to directly commit to the pull-request branch.

What did happen:
Pull request pipeline only ran the default steps and ignored re-running the PR steps for the branch.

Bug:
As a user, I expect the pipeline to re-run the pull-request steps on a branch if an update has been made from merging a different branch to it.