Any idea why it only leaks while sitting? Then goes all at once like this?

I replaced all of it with Mercon SP

 

This article points to the vent being stopped up on the Ford 6R80, the close sister to the 6HP26:
Solving the 6R80 Leak Mystery - Transmission Digest

At the bottom of this page, there's a link called "Repair manual 6HP26 ATSG". Download that book and look at pages 51-52.
6HP26 Transmission parts, repair guidelines, problems, manuals (go4trans.com)

Both of these articles seem to describe your problem. The solution on the first article is to fix the vent. The second article describes making sure your fluid level is right, and that your not sucking air through the filter seal or pump.

Either way, running it until warm or full temp, then opening the fill hole might give you the answers you need. If there's pressure in the case or a bunch of aerated fluid, either condition could be the cause.

Good luck!

 

did you ever diagnose this? I have the same type of leak going on