I recently said “thanks mate” to a blatant MtF person that works for the same organisation as I do. I probably very visibly shit a brick and thought “not your best choice of words”. If it’s obvious, I will use gendered words, if someone has pronouns or something to make it obvious how they want to be referred to, I’ll use them. If it’s not obvious, and they have nothing to indicate how they want to be referred to, I’ll just be polite.
Mistakes happen, be polite and apologise and I reckon you’ll be fine.