Speculation time!!! This is probably wrong! But I like been corrected :D :chop:
Is the blunt spot about 1 inch in from the heel?
This is a guess but if you put the razor on a glass block I suspect you have a high spot either on the shoulder, or where the shoulder meets the spine. Or a high shank. That's causing the blade to change angle mid stroke.
Try using 1 layer of tape, Applied so it starts 1/8" before the shoulder, And when you hone, Hone heel forward at an angle so none of the shoulder touches the stone at all.
If the spot is 1 inch away from the toe it could be you dropping your elbow as you hone. This will twist the razor into the hone and try to add a partial second bevel as you finish the stroke. To remedy is sit straight backed as you hone and move your hone to an angle where your elbow doesn't need to move for the stroke.
I suspect the reason we are not seeing the second bevel in full is the 4K has removed enough material that the 8K side simply isn't touching that spot at all.