It seems that the Interval context is not set up well to work with the AlternateInterval context. The context::AlternateIntervals::Parser::Interval package derives from the Parser::List::Interval package and the _check method of the context::AlternateIntervals::Parser::Interval calls the _check method of the Parser::List::Interval package. The parent method has the parentheses delimiters hard coded in the check that the infinite endpoints are open. That check fails for the alternate notation.
I recommend that you file an issue about this at https://github.com/openwebwork/pg/issues.