Well, I'm using Architect 2.1.0 Build 678 with ExtJs 4.1.x
I think I've found what's the problem here.
When adding a param or a new function, I did not validate the first param (the one I want to delete) by 'bluring' field (with ENTER KEY or click outside the field). Also do not validate second function name.
Have a look at the attached screen recording.
Thanks for the update and the description - you're spot on with the blurring issue - not tabbing off of those fields does seem to be the cause.
Fortunately, I can confirm that on our latest codebase, this is fixed and will be addressed with the release of Architect 2.2. Thanks so much for helping dig deeper into this one - it makes it a great deal easier for us to improve architect.
Sencha Architect Development Team