Hi. I'm posting this to know if this is a known limitation.
Short description of error
When you change the value of a shape key for a mesh that is deformed by an armature that has normal bones as well as bendy-bones, the mesh vertices will pop.
Exact steps for others to reproduce the error
Create a mesh and add an Armature modifier to it, with an armature object that has some normal bones as well as one or more bendy-bones (more than 1 subdivision). Use auto-weights or manual weight painting. Add and use a shape key on this mesh to see this behaviour.
This happens whether you're in animation playback mode or not.
I originally posted this as a Right-Click Select entry: