Run the check
Press each side button in the button test and note whether the page navigates.
Check the button assignment in the mouse software without changing several bindings at once.
Try an application that accepts those buttons, then compare another USB port.
Read the result
An event in another app but not this tab points to browser interception or mapping.
Limits and next step
A silent web page cannot prove a side switch is defective.
Repeat the check after changing one setting or connection. Keep a record of what changed before deciding whether the device needs service.