All three alarms I set on the Pro worked just fine. The three alarms were set after 2, 3, and 4 hours, respectively, when the input switched to Line-in and the Pro's LED lit green.
In your case, there may be a problem with your device, app settings, or network.
I don't know if this is related to your problem, but you should also make sure that the settings in "WHA Device Settings > Speaker info > Audio prompts Laungage" match your country.
Factory reset WiiM
Uninstall the app
Restart your router
These are also worth trying.
Sorry, I don't have any other solution. Submit a support ticket.