I’m not quite sure why the force set bonus isn’t working in this case, we’ll add it to the list to look at.
The main problem is that the 4pc bonus isn’t very strong, and certain stats are significantly worse than others for BM, so poorly itemized set pieces aren’t going to make that 4pc bonus worth it, unfortunately.
I think that the force tier set option is only not working on your “BM Drachen” setup.
The reason is a little obscure… it boils down to the fact that your locked trinkets conflict with your settings. You have locked an on-use trinket but then set Max On-Use Items to None in your settings.
Ultimately this gives the optimizer some trouble… it is being forced to produce sets of gear that are technically “invalid” according to your options, and then certain settings have to be relaxed or ignored internally to get the optimizer to run to completion.
Set Max On-Use Items to 1 or more, or unlock your trinkets and the issue should resolve itself.