This problem only comes with the new VFB. Try switch back to the old one (should be done on all your render nodes), and it solves the flickering.
Also some additional info we've noticed:
- Bloom and Glare computed on the slaves, but not burned in to the elements
- Sometimes the Beauty element has the B&G burned, and the Lightmix element do not, and vica versa, same with the B&G element
- For us it seems the problem occured during saving the separate elements