It looks like the initial transaction requested through /getTransactions is not in the masterchain (workchain=-1). You can try using /getBlockTransactions to look for the transaction in the basechain (workchain=0).

First, use /lookupBlock to find the block number corresponding to the transaction's Unix time, and then use /getBlockTransactions to search for the transaction in the block with the corresponding sequence number in each shard.

In the specific case mentioned, the transaction was found in the basechain (workchain=0, shard=-9223372036854775808, seqno=21407484). It's worth noting that the masterchain and basechain are separate blockchains within the TON ecosystem, and not all transactions will be in the masterchain.

Additionally, regarding the question of why there is only one shard in the masterchain for any seqno, this is because the masterchain only has one shard, whereas the basechain has multiple shards.