I've been using Jira automation rules and GitHub to track the development process on my team, and we've been struggling with this issue.
At the moment, we have a rule that works only when one issue is linked to one pull request (see below), writing the issue key on the title of the pull request. But my objective is to make a rule such that "When a pull request is merged, all the issues that are linked to that pull request transition to the status 'Ready For Build'", since sometimes multiple Jira issues/development issues are addressed on a single branch and then a single pull request is needed.

From the question "Update multiple issues, that are not linked, using Jira Automation" the solution seems to be using a branch action and a JQL query (see the second figure below). But the thing is that I've not been able to find a way to detect all the issues associated with the merged pull request using JQL. Is there a way to do this?

I've already checked that the pull request linked all my issues. In my case, the branch name was "WF-1009-WF-542-WF-635-model-plot" and the pull request title was something like "WF-1009 WF-542 WF-635: fixes of ..... #49", so the pull request #49 is correctly linked to the issues WF-1009, WF-542 and WF-635. Furthermore, every issue has the branch "WF-1009-WF-542-WF-635-model-plot" associated.