Those IP addresses are from the Let's Encrypt staging environment. Something specific to the staging environment is that it performs validation from multiple servers at the same time (Validating challenges from multiple network vantage points).
There are three remote VAs and two of the three remote VAs must succeed for the validation to succeed.
Right now, I see 4 requests arriving for a single challenge. I'm not sure if the above statement is still true about the required threshold for success, but I think it's the most likely explanation for your symptoms.