Multiple debuffs of one type are only shown once
Summary of the issue:
When a player gets multiple debuffs of the same type (for example Spit in Waycrest Manor) VuhDo will only show a single debuff independent of how many there are. When deciding who to heal this is critical information not available. All debuffs that give multiple instances of the debuff do their damage individually so there's no reason for them to be hidden.
Debuffs that can applied multiple times are shown individually.
If more than one debuff of the same type is applied only the first one is shown.
Easiest way to reproduce is to go into Waycrest Manor, engage some Maggots while having all debuffs shown enabled and then look at the frames.