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I had the opposite experience a few years ago: I was a new user of password managers and did not let mine autofill when booking my plane.

The air carrier form was not consistent with my country's name nomenclature: First, Middle, Last instead of First (+second and third) and Last name. I filled it wrongly and had to pay $40 at the check-in counter because there was no passenger registered to my name on the flight.

Tried again with my password manager (Dashlane) a few days later and realized that I should have trusted it. Also, I know that it only fills fields after the selected one, not those before.