Same here.
In fact, it would be nice if this was user-configurable. Currently, it appears that 1Password decides if an "Login" item can be suggested for SSO by checking whether the associated website is in a short list of "known" IDPs. (And this list currently doesn't include login.gov...)
Instead, how about a having a flag that can be set on the "Login" item that tells 1Password that is should be suggested as an IDP for "Sign In With". With these custom IDPs, 1Password wouldn't need to actually do the OAUTH process, just remind the user what IDP (and account) they used.