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.