I haven't tested this exact use case (TextArea with MaxLengthValidator), but I have at least one case that is working with both a) validation and b) auto-validate:
http://www.sencha.com/examples/#Exam...e:basicbinding has a TextField with a RegexValidator, written like this:
The regex is a little confused - it only checks that there is at least one non-lower case letter in the field - but this can still be used to test. Empty out the Symbol field, and type just the letter a. This will cause a validation error to occur, and adding any capital letter (or any number, or punctuation, etc) will cause the error to disappear. You do need to wait for a moment to allow it to appear, but it isn't required to blur.
symbol = new TextField();
symbol.addValidator(new RegExValidator("[^a-z]+", "Only uppercase letters allowed"));
There is no special handling for RegExValidator that doesn't happen for the MaxLengthValidator as well - can you post a test case of this bug that implements EntryPoint, and details of a particular browser?