Growing commit list on each pull request having squash merge strategy on master branch

I have two branches: master and staging. The master is write protected, allows only pull requests and the merge is made using the squash strategy.

Given that, when creating each new pull request, the automatic description of the pull requests contains LOTS of commits from the staging branch. They were merged already, however, since they do not exist in the master branch (each merged pull request results in a single merge commit) the list keeps growing and growing. On each pull request we have to clear that description and leave only relevant commits which is a tedious work.

Maybe we're doing something wrong? However this sounds like a bug ;/