SMU left the test running on the v6 connection for the entire night, and no instabilities had been observed. SMU then re-enabled all the client IPv6 ranges in the morning. No issues so far.
In the meantime, we will continue to investigate why IPv6 link-local addresses must be allowed in the filter.
Issue was related to the Content Delivery Service migration work Friday Sept 1 (TicketID: 20230826-001). The existing policy inherited during the migration Friday has been operating for more than a year and however we required an explicit change to the policy in order to restored service stability. We are looking through vendor knowledge base to determine why in this particular case explicitly including the link-local IP addresses (FE80::) was required however in the mean time the instability has ended due to the policy update.
SMU has reported that on Monday they lost access to Google and Microsoft over IPv6 routes, significantly impacting their internet servicese.