Triggers with only one rigid body

Hi all, I'm getting no trigger events with the following setup: I have one collider 'A' (IsTrigger, rigid body: IsKinematic) and lots of other objects 'B' with collider triggers (no rigid body).

According to the Collision Matrix I should be getting trigger events with this setup (static collider vs kinematic rigid trigger).

I do get the OnTriggerEnter event called if I create one of my 'B' objects inside the rigid body trigger 'A' (presume there's a different rule that fires this!)

I also get all trigger events called ok if I add a rigid body to the 'B' objects, though there may be a lot of these and I don't want the cost of all those extra rigid bodies in play (and I shouldn't need them according to the collision matrix).

Have I got something wrong here..? Searching here suggests some strange things going on with triggers but no-one is quite sure if it's a bug or not.

[Unity v412]