It looks like in some circumstances, cargo will still attempt to perform network lookups even when using a registry replacement. One such case appears to be git-type sources that specify a branch instead of a commit, even if that depdendency is present in vendored sources.
Should be fixed with https://pagure.io/fedora-rust/rust-packaging/c/61c353b247f39ae90f4b46021a3ae4ce0f6a6a2e?branch=main and was released in v26.1.
Metadata Update from @decathorpe: - Issue status updated to: Closed (was: Open)