The question is, do we need to run another build and test cycle after this merge or what?
That merge shouldn't break anything because it should be a fast-forward merge, all the commits on master are on the release branch. Therefore, you can not create a bug on master post-merge that wasn't on the release branch.
So technically yes, it isn't the precise commit you built, but the philosophy is that everything on the master branch is in production. At anytime, if someone pulls the master branch he should get the current production code. That's why you don't merge and then build, and test, and wait, and fix things on master for a release.
Now things does not always go smoothly. By the time the release has been validated and is ready to ship, you may have encounter major production bugs that needed a hotfix, in that case some commits have been pushed to master and develop but not to the release branch. If this happens, I'd rebase (be careful with that when working in team, merge is safer) the release branch on develop (to catch up with the hotfixes) and rebuild again. To sum up, if there are no hotfixes between the time the release branch was created and the time it is validated, no need to rebuild.