I have the same devices (Flex, which I’ve had for a while, and a new Node 2i.) I’m experiencing the same grouping/syncing problem with the two Bluesound devices. They seem to go out of sync after hitting the top panel play/pause button on one of the devices (and maybe after other events too).
My work-around: redirect the amp (or other output device) away from the Node and directly onto the Core server. In my case, that means switching my amp input from coax (fed by the Node) to USB (fed by a Mac Mini that runs the Core). I’ve been grouping the Flex this way with no apparent sync issues. The musical output is the same as what I’m trying to achieve with the Node+Flex grouping … except I lose the ability to control through the Node whenever I want grouped playback. The other work-around would be to group the Node+Flex, but just stop doing whatever triggers the out-of-sync state.