No, Let's Encrypt does not publish a list of IP addresses used by its validation centers. This is a good wiki article that explains the details: Multi-Perspective Validation & Geoblocking FAQ The entire wiki is good but I link to the section more directly related to your question.
Can't you view your own DNS Server logs to see what was blocked?
Do you have any kind of limit on the volume of requests inbound to your lone DNS server? There can be a large burst of queries during a challenge. See for example what happens when looking up records using https://unboundtest.com
Note unboundtest does not run inside an LE validation center so cannot mimic the multi-perspective validation (or the total volume from multiple simultaneous centers).