I have OPNsense 24.7.10_2, and I have two gateways set up in a gateway group, with WAN being the primary (high priority) gateway and WAN2 being the secondary (low priority) gateway that is only used through failover when WAN is down.
Today, WAN2 was down most of the day due to an outage at the internet service provider, and it caused some, though not all, websites to fail to load on WAN using Firefox, Chrome, or any other browser. Examples of the websites that failed to load are cnn.com and nytimes.com. Strangely, however, I could still ping those websites despite not being able to access them in a browser. When I disabled WAN2 under system, gateways, configuration, however, all websites loaded properly on WAN.
In sum, there is a bug in gateway groups such that when one of the monitored gateways goes down (in this case, the secondary gateway), the other gateway does not work properly.
This is a continuing issue with gateway groups and monitoring. They didn't work properly in 24.7.1, but the problem was corrected in 24.7.3. See, for example, this post: https://forum.opnsense.org/index.php?topic=41915.15.
Now, it appears that in 24.7.10_2, there are still some problems. In particular, if the secondary gateway goes down, some remote hosts are inaccessible on the primary gateway using http or https.
If anyone has any insights or solutions, it would be great to hear them.