This is a known issue. It has been broken since before ARCHICAD 19, but rarely anyone uses special windows with ganging, so they have not been reported widely. I am not sure when they will be fixed (issue #180779).
Anyhow, I think you are better off if you just put together your windows with the ganging turned on, and the casings turned off, and then place the casing on it manually with a morph. You can also save custom windows if you nead really special shapes (model your window in the floor plan view, and use 'file>libraries and objects>save selection as...>window').
Daniel Alexander Kovacs
Professional Services Consultant
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