You are correct to be frustrated. The new Windows App relies on "Identity-First" auto-discovery which frequently fails in complex B2B multi-tenant scenarios where the manual "Subscribe with URL" feature was essential. Although the Classic Client is being deprecated, the hard End of Support date is not until March 2026, meaning you have over a year of safe, supported use. The new app fails here because it assumes your home identity will automatically advertise your guest links, a process that breaks easily under specific Conditional Access policies or tenant configurations. Since you confirmed the Classic Client works perfectly, the robust technical solution is to keep these specific users on the Remote Desktop (Classic) client (MSRDC). The new Windows App currently lacks the feature parity to handle your cross-tenant setup reliably without the manual "Add" options. If you are forced to use the new app, the only potential workaround is signing out completely and attempting to "Switch Organization" via the profile menu, though this is often inconsistent. For now, treat the Classic Client as the production-ready tool for this specific use case and delay migration until the new app matures.
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VP