See implementation: https://source.chromium.org/chromium/chromium/src/+/main:content/browser/renderer_host/navigation_request.cc;l=1314-1330?q=%22Navigations%20cannot%20be%20initiated%20from%20a%22&ss=chromium%2Fchromium%2Fsrc

The behavior is if you have a nested iframe inside a fenced frame. The fenced frame disables its untrusted network, then attempts to start an embedder-initiated navigation of the nested iframe. The check linked above will be hit and the navigation will not be inititated.