Let's Encrypt uses multiple vantage points to validate a hostname. That's what is meant by "secondary validation" in the error message. See for more information:
- https://letsencrypt.org/2020/02/19/multi-perspective-validation.html
- Validating challenges from multiple network vantage points
So you're seeing only two of the four (one primary and three secondary) validation attempts. This means 50 % of the validation attempts are blocked somehow.