Nothing you say during a meltdown is being processed
The instinct to explain during a meltdown is strong and almost always wrong. The child is not refusing to listen; the machinery for listening is offline.
A child at the peak of a stress response has reduced access to language comprehension and almost none to reasoning. Sentences arrive as noise and tone. Complex ones, or raised ones, add load to a system already past capacity, which is why explaining often makes the episode longer.
What works instead
Presence, fewer words, lower pitch, and waiting. The full sentence — what happened, what to do next time — is not lost by waiting; it is the only version that will ever be heard.
The conversation is not cancelled. It is postponed by ten minutes, and it works then.
The adult's half
Waiting is unpleasant, especially in public, and most escalation comes from the adult's discomfort rather than the child's behaviour. Knowing that in advance makes the ten minutes easier to hold.