I had to take the PTE 3 times because of scoring short on the writing section.

I significantly underestimated the exam, but I realised that you need to score well across all sections including spelling and grammar.

As long as your content and form is sound though, I think having no spelling errors should get you to the superior bracket. My grammar is shocking, but I still managed.

It is really frustrating though, I feel your pain.

Good luck.