Tody, IPv6 monitoring is often forgotten, ignored or done from the wrong vantage point.
Some examples from experience:
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Large corporate network with no IPv6 monitoring, because the part of the network where the monitoring system was located had no IPv6.
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Web services with AAAA records [2. DNS] and proper configuration; monitoring said that everything okay, but users could not access the web services via IPv6 from the Internet. Someone forgot a firewall rule, and the monitoring system was on the inside of the network.
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Mail (SMTP) server with AAAA records. However, IPv6 was disabled (or blocked by a firewall) for whatever reason, but nobody removed the AAAA records. Wasn't noticed internally, i.e. they did not monitor via IPv6.
There is no fundamental difference between monitoring services for IPv4 or IPv6; it just has to be done for all services and, if they are dual-stacked, for both protocols.
In case of the mail server example above, there were probably three different teams involved and they either didn't talk to each other or had an inadequate process implemented and no automation.
Related to this, implementing IPv6 also gives an operator the chance to clean up operational documentation, ops infrastructure and NOC processes. It may also be an oportunity to implement more automation.