If you want to be sure that the transaction is included in a block, you can subscribe for the result using JSONRPC via a websocket. See https://docs.cometbft.com/v0.34/core/subscription.html If you haven't received anything after a couple of blocks, resend it. If the same happens again, send it to some other node. A few reasons why it could happen:

1. malicious node can drop or pretend it had committed your tx
2. malicious proposer (not necessary the one you're communicating with) can drop transactions, which might become valid in the future (https://github.com/tendermint/tendermint/issues/3322)

Please refer to https://docs.cometbft.com/v0.34/core/using-cometbft.html#formatting for formatting/encoding rules.