I was under the impression that eth_newPendingTransactionFilter returns pending transactions that are part of the `txpool`. However, in my recent experiment, this doesn't seem true.
Here is what I did. First I connected to a full node and start listening for a eth_newPendingTransactionFilter. Then I sent test transactions to the full node. Third, I made sure those transactions appear in the txpool by calling txpool.inspect. I expected they'd also show up in the filer, but they never did.
What could be the possible reasons?