When a CrOS build is made on Linux (with gn arg target_os = "chromeos"), an experimental arm enabled by forcing_feature_on won't be enabled by a chrome://flag. It *will* be enabled with --enable-flags=FeatureFlagName, however.

I think (though I'm not certain) that the issue is that in this setup, chrome://flags are not copied to the command-line because this kind of build uses a stub version of CrOS's SessionManagerClient that returns false for SupportsRestartToApplyUserFlags():
https://cs.chromium.org/chromium/src/chromeos/dbus/session_manager_client.cc?l=1091&rcl=2066573c42bfd254d93ca5801998f65abd39b121

Given that there are lots of other conditions under which chrome://flags are not copied to the command-line:
https://cs.chromium.org/chromium/src/chrome/browser/chromeos/login/session/user_session_manager.cc?l=757-782&rcl=3dc8ae2e46895de7953b0427e40817887135270b
It may be worth implementing some other mechanism for applying the state of chrome://flags to experiments.