You can use an Arch BuildingPart precisely to replace the simple Std Groups. A BuildingPart is not meant to be a specific room, or floor, but to group any type of architectural objects. At least, that's how I understand the tool.
This is probably a bug. However, maybe that functionality was just added to 0.19, and you are still using 0.18 it seems. So, if you could test 0.19 and confirm that it works as expected, then it means this is only available in 0.19.
FWIW: In an earlier version of V19 (predating the mentioned recompute issue) this does not work either.
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OS: Windows 8.1 Word size of OS: 64-bit Word size of FreeCAD: 64-bit Version: 0.19.17337 (Git) Build type: Release Branch: master Hash: 0871fb49c71f1ac4029e441814332368f42bd78a Python version: 3.6.6 Qt version: 5.6.2 Coin version: 4.0.0a OCC version: 7.3.0 Locale: Dutch/Netherlands (nl_NL)