If gateway monitoring is active for an ipv6 gateway with an ula address, then the gateway or the service must be restarted after the opnsense restart.
If gateway monitoring is disabled, then the service or gateway starts normally.
The problem occurs only with the dpinger.
OPNsense 23.7.6-amd64
FreeBSD 13.2-RELEASE-p3
OpenSSL 1.1.1w 11 Sep 2023
In had a similar issue that could be related. In the referenced github there's a workaround.
Issue seems to arise on statically assigned IPv6 adresses.
https://forum.opnsense.org/index.php?topic=36277.msg176884#msg176884
ULA is a bit of an issue if you want to use a monitor address that is a GUA...
https://github.com/opnsense/core/issues/6939
Cheers,
Franco