Ups, yes your signal is detected as supported one
Interesting problem.
You can solve it, but you need correct BM card- the one which does support hardware scaling. In this case you set resize in card settings and then any UHD signal is resized to HD. Only top end models support it though (still cheaper than AJA).
Decklink 4K Pro does this:
Colorspace Conversion
Hardware based real time on output.
3D Processing
Left eye/right eye dual stream capture & playback.
Ultra HD to HD Down Conversion
Built in, high quality hardware down converter outputs Ultra HD to HD or HD to SD down converted signal on SDI B. Down converted SD selectable between letterbox, anamorphic 16:9 and center cut 4:3 styles.
so you get UHD preview on 1 SDI and HD downscale on another.
No idea if it will scale 4K DCI though Other solution is some converter box which can pass 4K SDI and output scaled HD SDI/HDMI. AJA has one, probably BM also. This may be cheaper option.