BP seems to stop regenerating only when someone crouches, stays in place and doesn't hold block, all at the same time.
Blocking and/or moving while crouching resumes regeneration.
I doubt this is intended, but if it is, please correct me.
It doesn't say anything about plain crouching. Attacking, taking hits, or doing neither – simply crouching stops BP regeneration, and only that alone does. If you try to move while crouching, BP starts going up again.