In the heart of a meltdown, acknowledge how your child feels with words. For example, I can tell you are upset because you weren't expecting this (your personal example). Verbally confirming the reason for the child's frustration helps the child to feel they are heard, and calms things so you can work towards resolving the situation.
Autism Spectrum Disorders can be...well confusing. No two children are alike. No two cases are the same. Doctors are still figuring out the ins and outs of the spectrum. Misinformation is plentiful. It can definitely be a beautiful mess to navigate.