My ISP has an issue with some of their edge routers where they are not sending periodic unsolicited RA messages. This is happening with their Juniper edge routers. For their Alcatel/Nokia edge routers, unsolicited RA messages are sent every 20-30 minutes. The Juniper edge routers will respond to RS messages.
I'm not sure how this behaviour started. It may be that Juniper or one of their customers decided it was a feature. However, recently, Juniper started offering "no unsolicited ra" as selectable "Enhanced Subscriber Management feature". Here is a link: https://www.juniper.net/documentation/en_US/junos/topics/reference/configuration-statement/no-unsolicited-ra-edit-enhanced-universal-edge-overrides.html (https://www.juniper.net/documentation/en_US/junos/topics/reference/configuration-statement/no-unsolicited-ra-edit-enhanced-universal-edge-overrides.html).
The behaviour of the other *sense is inconsistent. Sometimes IPv6 will drop after 2 hours. If you save / apply the WAN I/F, it will restart. Other times, it will continue to work.
What is the behaviour of OPNsense if the edge router doesn't send an unsolicited RA? Is there a way to trigger OPNsense to send a solicited RS message for situations such as this? I would try this myself, but it's working properly for me.