Looks like we can't reproduce the issue or there's a problem in the test case provided.
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I found a bug with Ext-JS 4.1.1 having to do with fieldset collapse. I'm posting it here because I'm providing an Architect archive file that demonstrates the problem. I don't expect that it has anything to do with Architect itself.
I tested this with the latest version of Chrome running on OSX.
Deploy the example and collapse the field set labelled "My Fields". You will see that the panel title is affected and the content of the other field sets on the page are hidden.
In my real application I found that the entire contents of the panel were hidden. Slightly different outcome, but I believe probably the same bug.
This happens when the minHeight of the first field set is set to a value (in this case 50). If minHeight is not set then everything works as expected.