I tried your example and it is working perfectly fine. What i could guess the problem might be is, that you might be using AdobeSpellingUI.swc for Spell Checking. Spark components are not supported by AdobeSpellingUI.swc. For Spark and Mxml components squiggly offers AdobeSpellingUIEx.swc. Please use AdobeSpellingUIEx.swc and let me know if the problem is resolved. Also please make sure that the dictionary files are in the correct folders as specified in AdobeSpellingConfig.xml
Please refer to Squiggly 0.6 AsDocs and Release notes for more details.
Your assumption is correct. After I removed the AdobeSpellingUI.swc from the libs folder, all works.