I found this (possible) bug when making a role-specific channel in my discord. I was making a "movie night" channel that only "movie watchers" and "movie curators" can see, and only "movie curators" can post in. Importantly I wanted both of these roles to be able to add new reactions to posts. This is how I had the channel's permissions set up:
@everyone had all permissions disabled except "Mention @everyone.." "use External Emojis" and "Add Reactions" (all three green check-marked), and watchers and curators were able to "Read Messages". A curator could post but importantly could not add any reactions to any posts. I had redundant green-checkmarks on the roles to ensure they have the permission.
Only when I enabled "Read Message History" on @everyone could anyone (besides admins) add new reactions. Enabling the history allowed anyone who could see the channel to react as I wanted. Is this a bug or is there some logical reason why adding new reactions to messages should require message history?