Traffic loss is expected when you upgrade a single leaf that is not in an MCT pair. Because there are no alternate paths for the single leaf, maintenance mode is not enabled. Only the configuration is persisted, and a firmware upgrade is carried out. A traffic loss warning is flagged when you upgrade a single non-MCT leaf.
Traffic loss is also expected for any singled-homed server. Detecting single-homed servers are not in the scope of this feature so a generic warning is provided at the start of a firmware download.
This is not a typical deployment, but traffic loss is expected in this scenario. Because no alternate paths exist for non-redundant devices, maintenance mode is not enabled for this case. A traffic loss warning is flagged when you upgrade non-redundant devices.