A bone parented to another bone, can't drive the parent bendy bone values (easein, easeout...) without creating a dependency cycle (resulting in one frame lag, plus crashing Blender when many bones are setup this way). Since it is a common scenario when rigging characters, to add tweak controllers on characters, and it used to work fine in Blender 2.79, I would strongly recommend to consider it as a bug instead of a "feature". It would be a pain to rig without that working properly, and would totally break existing rigs from Blender 2.79.
The left bone is parented to the right bone. It's driving its bendy values.
Open the console, a dependency cycle is detected.
rBafa3aec45d95: Fix T59734: cyclic dependency with drivers on PoseBone.bbone_*.
Alexander Gavrilov (angavrilov) triaged this task as Confirmed, Medium priority.
Alexander Gavrilov (angavrilov) added a commit: rBafa3aec45d95: Fix T59734: cyclic dependency with drivers on PoseBone.bbone_*..