So sorry to hear about the little guys. As Bruce said take out any anubias that seems to be affected and brutally cut the soft part of that rhizome. The rhizome(part from which roots and leaves come out) should be hard to the touch.
I can think of two reasons why this might have happened. 1... The plant were planted to deep or crammed into a corner with no problems little waterflow. Try moving the remaining plants off the substrate onto a piece of wood or rock, into open water, away from other plants. 2... You Have a macro missing. Did you add a phosphate or nitrate or other 'removing ' media to your filter?
Can you think of the answer for this question:
What changed from the time when the anubias were growing? (2-4 WEEKS ago maybe)
On hiatus till later this year