Updated whole issue after new version of bitbucket-branch-source plugin

We use the Bitbucket branch source plugin with a private Bitbucket server instance.
We create the pull requests of our features branches quite early to see, if the pull request merge can be build based on our Bitbucket pull request merge strategy. So we want to build the pull-requests/*/merge refs.

An example repo, where git ls-remote shows the following refs:
 9e4f3f544c1595d5fa23b0acc6480abe852f8494 HEAD
 dac124e3e1a73d2a9a10d7a99ad79d733ada6db3 refs/heads/feature
 9e4f3f544c1595d5fa23b0acc6480abe852f8494 refs/heads/master
 dac124e3e1a73d2a9a10d7a99ad79d733ada6db3 refs/pull-requests/1/from
 3219b86ea2902d63d983d849f2c95f288587401a refs/pull-requests/1/merge

The different discover pull request from origin strategies for the pull-requests merge produces always a checkout of refs/pull-requests/1/from and perform a local merge.

It seems not possible to checkout the merge ref directly. Is there a reason for this behavior and how can can this be configured? Because the local merge is done with the default merge settings and not with the one configured in Bitbucket.