I realize this is a difficult, if not impossible, question to answer without seeing it but after sealing the ENTIRE surface area and all the seams and sides inside the gutter using the method and products similar to what's shown in the following video, there's STILL a small leak that's trickling from below and behind the gutter in one place.

https://www.youtube.com/watch?v=acxYPweeDOE

I was very careful about making sure every inch was covering with the sealant and tape while the gutter was dry so I've reached an impasse.

One thing worth mentioning is that this particular area has standing water because it's not quite pitched properly. It eventually drains away from this area but slowly. This pitch obviously needs to be fixed but still wondering why the patch didn't work. Maybe a naive question but does standing water/bacteria, algae, whatever, just wind up working it's way through ANY patch job?

 

Obviously there is a void in the patch somewhere, maybe the patch didn't extend out far enough and water is getting under it. Are you sure the gutter was good and clean before you started?

 

The back of the gutter must be exposed (not covered by roof flashing).

 

You've got me wondering if that's the issue. The back of the gutter is indeed covered.