Ribbons that disable Inherit Parent Normalized Age flicker off when one ribbon reaches the end of its lifetime. This seems to be tied to the texture coordinates used in the default material, and offsetting the UVs by age. Removing the TexCoord node in the ribbon material or specifying a UV0TilingDistance appear to prevent the flickering.
All other ribbons will flicker off.
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-85871 in the post.