IIRC the 2011 and early 2012 blocks had some issues with warping? Not saying that's it for sure, but was a known issue; however that typically caused a "ticking" noise. Other possibilities are broken ring(s), valve spring, etc.
Too early to know until you get the diagnosis back.

I'm not really sure how a computer can test for "compression". I guess I'm old skool and believe in the old fassioned way by hooking up a pressure gage to the spark plug hole. A computer can interpret the knock-sensor signal to determine a "misfire", but "low compression"? Not sure how a computer or electronic sensor would know this. The "compression" does not change with firing of the cylinder (pressure will change on the firing downstroke, but not the "compression"). Compression is a function of the sealing of the combustion chamber; rings and valves and cylinder walls, barring any cracks in the head.

 

The coyote is known for having over heating issues with # 8 and 4 cylinders. Modular motor sports racing offers a super simple fix that takes care of the problem. The part # is 455478 it's a simple bolt on, the problem is it bolts on to the back of the cylinder heads. Just pop out the freeze plugs bolt in the crossover bungs then connect with a rubber hose, super simple just hard to get to.

 

Interesting, but a few things I'd note about this comment you make ...
Looking at their website: MMR 2011-2017 Mustang GT 5.0 Head Cooling Mod HCM [455478] - $199.99 : Modular Motorsports, Home of the Worlds Fastest Modular Engines

It seems a few things are not probably applicable here:
1) Mustang application (tighter engine bay = less heat dissipation)
2) Mustang application (higher specific power output and also higher rpm rating) make for more heat to begin with, versus the F150 de-tuned version
3) This mod you suggest is for " ... cylinder # 8 - the cylinder long known to be the first to detonate with aggressive naturally aspirated tunes and forced induction."
4) You mention cyl 4; they don't

I don't think this is a concern in F150 Coy motor if one keeps it stock.
Sure; their website shows the kit available for the F150; it fits because it's a Coy 5.0L. They will sell you one because they want money. And if you aggressively tune your Coy, you may need this device.

So the reliability issue you mention isn't really applicable here; we're not talking about hot-rodded motors.
I have not heard of any cylinders overheating in STOCK F150 5.0L engines.

I am not saying the mod is useless; it seems to have merit in certain modified applications.
But I don't think it's fair to say the 5.0L Coy F150 has a continuing reliability issue, when "aggressive ... tunes and forced induction" is likely the cause.


Kind of like saying the SD driveshafts have a problem with reliability; you know - when you "tune" the 6.7L to 800 rwhp and then put 38" tires on it, doing burn outs to impress the ladies ... Must be the driveshaft's fault ...

 

I did make a mis-statement, what I meant to say was that coyote blocks do suffer from cylinder wall failures due to excessive heat issues, primarily in boosted applications, but heavy towing could have the same affect. The stock block does have paper thin cylinder walls, just offering a possible reason for the low compression on cylinder 8.

 

You may be right; I don't have enough experience with them to know.

I just don't see many complaints here about that type of issue, AFTER the blocks were modified in 2013. As I said, they did have some warping issues in 2011 and early 2012. At some point in 2012, I think they addressed this. So a STOCK truck after mid-2012 should not experience this, as far as I can tell.