SN (Brussels Airlines) seems unbookable for me currently. I've been trying intra-European segments, and get no results when in SmartSearch or *A, and an error when restricting to SN only. I'm seeing availability in every other *A award tool I can use (expertflyer, M&M, United, ANA).
It has worked previously. LM ticketed a BRS-BRU-YUL-BOS for me a couple of months ago (BRS-BRU is SN operated by BM).
It's all very messy and wobbly. It feels like they've bitten off waayyyy more than they can chew.