It's because of ACME v1/v2: Validating challenges from multiple network vantage points.
To get a successful validation, the primary and 2 out of the 3 secondary validation servers must be able to perform the challenge.
Per your error message, at least two of the secondary servers claim to have failed to resolve your domain.
It's definitely odd that you have 3x HTTP 200s there, though. It would suggest that only one secondary server failed.
edit: removed ping