Child Pays for Parent transactions get stuck on testnet I'm building a service prototype by reusing Electrum code and generated 4 consequent transactions which are CPFP type on the testnet. They are staying unconfirmed for several hours but test transaction with 10 sat/vbyte got confirmed in the nex block.

What could be a possible reason for that? Could it be testnet-specific so I should not expect the same behavior on mainnet?
The problem you're facing is that even with a high-paying child, the parent needs enough feerate on its own to get relayed on the network.

Staying below 1 sat/vB makes that hard.