This is because the "What is this?" answer takes precedence over the "Not a dissyllable." answer.

However, that's not clear from the Kata description, because strings like -!, match both error types and it doesn't specify the one to return in that situation.

I've updated the description to be clearer.

It's totally fine - looks like a good solution to me. Why wouldn't it be allowed?

Under the Not Poetry section, -!, and #x3 should fail as Not a dissyllable, but instead are expected to fail as What is this?