As I mentioned earlier, mempool can evict transactions - there is a size limit for the entire mempool and an expiration time for each tx. Though at this memoment it's unlikely to happen (the size limit is 300Mb and expiration time 2 weeks), it's better to fix the problem fully from the beginning. At this moment eviction only happens after a new tx has been added to the mempool. So the fix could be that whenever the wallet receives a NewTransaction event, it would obtain all InMempool tx ids from the output cache, check which of them are no longer in mempool and reset their status. (though it's better not to use MempoolRpc::contains_tx for checking and instead introduce an additional method that would operate on a set of ids, to avoid the rpc calls overhead).

I added a TODO for this