After reaching 99 stacks alone or together, whenever another person, but the skiff owner catches 1 more fish the icon in the buff bar will disappear for all that are affected.
as far as i noticed the stacks remain at the correct number, yet somehow, and i do not now how to reproduce this yet, it is possible to loose all stacks randomly after this state of "having 99 stacks but not showing any" is reached.
this is somewhat annoying, as getting to 99 stacks on your own, especially in areas that require high base-fishingpower can really take some time.