Unless someone in the community knows the answer, it might take until next week to get an answer from the Discourse team about this.
I see that the plugin runs a job every 5 minutes to process all objects (posts, topics, users, tags) that are queued to be indexed: https://github.com/discourse/discourse-algolia/blob/main/app/jobs/scheduled/update_indexes.rb. It’s not clear to me what would cause an object to be reindexed though.
If you have a chance to respond, I’m curious about what you are seeing that makes it seem that content is being reindexed.