Working with another user who has kindly provided a bug case I think I've isolated the change in behaviour. If you're moving a static collider trigger (a collider not attached to a Rigidbody2D with IsTrigger checked) then you'll no longer receive callbacks for it.
In Box2D, you don't receive callbacks for sensors (triggers) until the next physics update (they are always delayed a single update). Previously we explicitly iterated all trigger contacts and performed a callback for each to ensure we do report callbacks immediately unlike stock Box2D. However, during a 5.2.3 patch, this changed slightly and this very specific behaviour was modified.
I am making a modification now that will revert this behaviour and add a test to ensure it never gets reverted in the future but again, this is related to static colliders only. You should not be moving them but it seems lots of people do even though it's expensive to do so. You should always add a Kinematic Rigidbody2D, again, something that is perceived as expensive from what I can tell.
I am supposed to be on leave this week but I'll get this into the release stream regardless as it seems to be affecting more than a few users.