It does and does not happen with 2160/24fps. Let me explain. I have a Yamaha 1079 and to this I have connected a UHD TV and a 1080p TV. If I have both video units connected through the 1079 (1+2 output on Yammy) the resolution of the HiMedia always shows 1080p, the lower resolution takes precedence. However, if I switch the Yammy to output 2 (the UHD TV) and set the resolution on the HiMedia to 2160/24fps and leave the Yammy setting on 2 it does not change resolution, Switch the Yammy back to 1+2 and the resolution changes to 1080p.
Ipso facto, the HiMedia Q5 Pro, in this case, is not at fault.