On regtest, I can use generateblock to efficiently a mine transaction i.e containing an enter with nSequence (0x05000000), though the enter has not reached the desired age (5 confirmations). Although I get non-BIP68-final (code -26) error once I use sendrawtransaction. Questioning why that is the case.

This is how I do it:

For simple demonstration I fund OP_TRUE (decodescript 51). It offers me p2wsh deal with: bcrt1qft5p2uhsdcdc3l2ua4ap5qqfg4pjaqlp250x7us7a8qqhrxrxfsqseac85.

Then I fund this deal with 1000 sats: TXID: a21c3ccc203f0c30c613e4409b011e5753ca10bb93fe5036849ab87e14e4cb6c, VOUT: 1.

Then I mine this funding transaction:

generateblock 2N1AwW2Jv4NcsgBWwggDPN7PLx4MpgRNjhs '["a21c3ccc203f0c30c613e4409b011e5753ca10bb93fe5036849ab87e14e4cb6c"]'

Then I manually assemble a uncooked tx spending this enter with witness 0x51, with nSequence 0x02000000:

020000000001016ccbe4147eb89a843650fe93bb10ca53571e019b40e413c6300c3f20cc3c1ca201000000000200000001f4010000000000002251209bb9efbddf9d70afd3ac2cef011747236bdf90832a78b08f57d1139f07aa918501015100000000

sendrawtransaction 020000000001016ccbe4147eb89a843650fe93bb10ca53571e019b40e413c6300c3f20cc3c1ca201000000000200000001f4010000000000002251209bb9efbddf9d70afd3ac2cef011747236bdf90832a78b08f57d1139f07aa918501015100000000 offers me the error: “non-BIP68-final (code -26)”

Nevertheless,

generateblock 2N1AwW2Jv4NcsgBWwggDPN7PLx4MpgRNjhs '["020000000001016ccbe4147eb89a843650fe93bb10ca53571e019b40e413c6300c3f20cc3c1ca201000000000200000001f4010000000000002251209bb9efbddf9d70afd3ac2cef011747236bdf90832a78b08f57d1139f07aa918501015100000000"]'
efficiently mines the block:

{
  "hash": "0f63ae01726730e186a5a8e16c2b4f9673ce135c46d033f8c60ee636319ffa6a"
}