From JVC NX5 manual:
When the color space is RGB/YCbCr(4:4:4), only 8-bit input is supported.
^ There's your answer. You will have to use 8-bit for 4k/24Hz.
systemctl stop kodi
./proptest -M i915 -D /dev/dri/card0 308 connector 314 8
systemctl start kodi
try if 4K/24 now works
Upd: the manual actually refers to 4K 50/60Hz modes, so deep color should be supported with 4K/24. Try also 10-bit (./proptest -M i915 -D /dev/dri/card0 308 connector 314 10)