I normally trace the problem from the source all the way to the end.
1. Can you check if you can press the scan/reset button on the back of the MST Hub to see if it helps?
2. Can you check if you can swap the HDMI cable to verify the cable is not the problem?
3. On the Samsung HE40A, did you check the picture modes for Screen Fit and Smart View1? You can check in the manual here
4. Auto adjustment?
In regards to the custom resolution 1080p 30Hz if I am not mistaken it is supported by the Catalyst driver. Also note that in the CCC there is an option to add custom resolutions if unavailable, add the required resolution with refresh rate and then apply it.
Note that HDTV has image post processing job so it decodes every frame sent to the TV, depending on each image mode, if the processor is not able to handle the job it will blank the image.
Also in the CCC there is a choice of color formats YCbCr 4:4:2, YCbCr 4:4:4 and RGB (HDMI Limited/Full Range). HDTV has support of the Luma/Chroma “YCbCr” on all refresh rates but not in RGB. Try YCbCr 4:4:2.
I hope this helps your inquiry.
Thank you for your support and we hope everything gets resolved.
AMD EDID utility please use this link