It cannot be “fixed” unless you centralize into a single firehose like Xitter and Bluesky.
Those of us running smaller instances can choose to use relays and fetcher-tasks - and there is a PR to put one such fetcher into Mastodon - but if the goal is 100% of everybody always sees 100% of the content then no decentralized solution will ever offer that.
That’s just replies to a post. It doesn’t solve not all posts reaching everybody.