I recently acquired the D54250WYKH and it also has a power button issue using the latest BIOS version (27).
After putting the NUC into sleep mode it no longer reacts to the power button, irrespective of whether deep S4/S5 is enabled or disabled.
The problem does not always occur immediately. It may be possible to put the NUC into sleep mode several times without problems before the power button ceases to respond.
Disconnecting or moving the on-board jumper does not fix the problem.
The problem is resolved by disconnecting and reconnecting the CMOS battery. Since that requires removal of the motherboard from the chassis this is not something that you want to do regularly.
Is Intel still looking into this problem to find a permanent fix?