Screenshots and log extracts, attached in order below. The two macOS user accounts are referred to as A and B. Both are on the same Mac and signed in to the same Apple Account, and both were talking to the same iPhone. A = my main account, where iPhone Mirroring fails. B = a second account I created purely for testing, where iPhone Mirroring works. 01 ACCOUNT A "Cannot Connect to iPhone" after setup runs. This is the failure. 02 ACCOUNT A A newly created Network Location active, shown next to the same failure. Included because creating a fresh Network Location was suggested as a fix and made no difference. I believe it cannot help, since a Network Location is system-wide while this fault is per user account, and B uses the very same location and connects fine. 03 ACCOUNT A Log extracts: the ScreenContinuityApp check sequence dying at step 5 with noCompatiblePhone, the replicatord errors (isCloudPaired: false, sync service does not know about it yet, no longer associated with account, unknownDestination, Reconciling devices Known: []), and the rapportd lines, alongside sharingd correctly identifying the phone as a SameAccountDevice at -42 dBm at the same moment. 04 A vs B Side by side table of the measurements taken minutes apart with the same iPhone. 05 A vs B The same comparison as raw log lines, showing isCloudPaired false in A and true in B, and Handshake completed in B. 06 ACCOUNT B On the iPhone, Settings > General > AirPlay & Continuity > iPhone Mirroring while B was mirroring. Note there is exactly one entry, the Mac's name, marked Currently Connected. This is what makes me think the pairing is stored per Mac rather than per macOS user account. 07 ACCOUNT B The iPhone's AirPlay & Continuity screen, showing iPhone Mirroring bound to this Mac and Handoff enabled. 08 ACCOUNT B "Connecting to iPhone 16 Pro" in the working account, the state A never reaches. 06, 07 and 08 were captured before I revoked B's pairing to free the slot for the test described in the original post. After that revoke, A still failed with the pairing slot completely empty, which is why I no longer think the shared per-Mac record was the blocker.