Azure DNS - consecutive DNS challenges break validation

[ohadschn]

It seems that after a few DNS challenges are made, and the TXT record contains more values, exceptions like this are encountered:

Challenge record was Pq-z7SLbwD7f_SXrA-J-akjsdhaskjdh-83D4 should have been fKESI1optQ-PjI2sYX41pC0akjsdhlaksjdh70L4, retrying again in 5 seconds

Looking at the actual record, both are there. So the code might be looking at the wrong one - I'm guessing because an assumption is made on the order of returned values:

letsencrypt-azure/src/LetsEncrypt.Azure.Core.V2/DnsLookupService.cs

Line 37 in 7d63cab

queriedDns = dnsRes.Answers.TxtRecords().FirstOrDefault()?.Text.FirstOrDefault();

When yet more challenges are made, the exception becomes (probably at the fault of the DNS client):
DnsClient.DnsResponseException: Unhandled exception ---> System.IndexOutOfRangeException: Cannot read that many bytes: '43'

Either way, manually deleting the TXT entry fixes the issue. Maybe you could delete it each time, rather than add values to it?