In that case, assuming you measure it correctly, and based on your information it should be a U6 5v side failure.
This can happen when...

One heavy load on the 5v circuit due a defective part on this circuit.
Or a defective U6 chip.

But could also be caused by a defective part on the U6 section, the 5v side.

The schematic of the u6 you can see by this picture,
http://jb8a8f8.com/support/index.php?attachments/u6-2012-jpg.28/

I advice to check all connection on the 5v side, se if there is no interruption.

Also, check the two (green) F1 and F2 fuses, check if they have the 5v, and on both sides.

Another thing you can do is to remove the R109 0 ohm resistor and see if you get the 5v back on the output from the U6 5v side.
If so, then you have a issue with another part what is connected to this 5v circuit.
But to be sure, remove the sim card if you do this.

But still, it's illogical if you have the pin 1 / 1.2v and pin 4 / 3.3v, while you don't have the 5v.
Normally, these two voltages are also not present by a 5v defect.
This makes the measurements doubtful.
http://jb8a8f8.com/support/index.php?attachments/dm800hd-fault-finder-pinout-2013-jpg.21