Reproduced these. Good catch. The last hit doesn’t hit unless you’re at 5m or less, and there is no movement speed buff at all while channeling.
In regards to the buff’s “bug”, i think that technically, it’s how it’s intended to stay, simply because of one reason: you can cancel the channel, and you’d still keep the 4 seconds left of the buff due to the passive. So the buff’s timer basically keeps resetting to 5s during the whole channel.