This is expected whent here is downtime, especially significant, works like this.
Posts you make if a server is federated with ours will be cached ont he other server when you make the post. All interactions are with the cached version but any actions like boosts are **pushed** to us so we can keep a record on our side.
If we are down any actions that need to be pushed to us retry a few times but eventually will die and be lost. That is likely what happened here, the boost or other posts you missed during that time retried and timed out.
I think there are certain tools from our end where we can go and fetch stuff we may have missed but since it can literally be on any post to the beginning of time that would be a massive effort and would be quite rude to other servers to hit them up that much. That said it can be done.
If its just a few over our downtime period it may be best to just call it a loss, but let me look into if there are any efficient options.