Most common issue on a 2011 6.7L with low compression is due to a dropped valve. The valve typically drops in pie-shaped pieces and should not affect vehicles built on or after 29 March 2012. When this occurs, a glow plug DTC is the most common to set (P067x) where X indicates the cylinder number (1-8). Remove the number 5 glow plug and inspect the tip. If the tip is cracked, damaged or missing, a dropped valve would be the most likely cause and long block replacement will most likely be necessary. Ford has a TSB regarding this issue, which is for direct long block replacement (when under warranty) due to low compression, a glow plug DTC and a damaged glow plug tip.
There are not many "non-catastrophic" issues that can cause a loss of compression from a cylinder unless recent repairs have been completed. If the injector or glow plug is loose, compression could escape, though this is far from being "common" especially on this model year 6.7L.
You may be able to speak with the dealership for assistance with the repair, or contact Ford CRC (Customer Relationship Center) regarding CLP (Customer Loyalty Program) if you are a long-time Ford owner. They may be able to assist with the repair some. Best of luck!!