If the Task corresponding the master branch PR is part of an Activity, you could record the backports as additional Task issues in the Activity.
If the Task is Maintenance task, and the backports are trivial or fairly straightforward, record the main PR and all the backport PRs as a task list in the Task issue description.
If the backports require substantial effort, the backporting could be worth of recording as an Activity, with each backport being its own Task.