Mute-switches of Multiband send turn automatically off when using render to sample without including the send track.
Seems like a bug to me, but if it’s a feature I really don’t see any benefit of it.
If there are specific frequency ranges (i suspect that is the purpose) that you want to have filtered, why don’t you use any of the EQ or filter devices instead?
Didn’t think anyone would guess how I’m using it 
Anyway - different filters than what filter2 offers
Well at least the default thought behind this behavior (and also disabling the mute buttons on send-devices) is that it is expected that you want to render the track as is without send-devices affecting the output of it causing confusion.
Although the multiband send offers some specific filtering purpose, it is still mainly a send-device and not a filter device.
This made me question if there would be a way to achieve the same effect using the filter or eq devices.
Perhaps there is a third party plugin that allows achieving the same or similar result without Renoise interfering with it as a solution for the time being.