`mempool_unbroadcast.py` test failure: `test_framework.authproxy.JSONRPCException: txn-mempool-conflict (-26)`

[whitslack]

Unit test mempool_unbroadcast.py failed:

2025-05-21T20:36:33.354000Z TestFramework (INFO): Initializing test directory /var/tmp/portage/net-p2p/elements-23.3.0/temp/test_runner_₿_🏃_20250521_195928/mempool_unbroadcast_28
2025-05-21T20:36:34.225000Z TestFramework (INFO): Test that mempool reattempts delivery of locally submitted transaction
2025-05-21T20:36:36.999000Z TestFramework (INFO): Generate transactions that only node 0 knows about
2025-05-21T20:36:37.261000Z TestFramework (ERROR): JSONRPC error
Traceback (most recent call last):
  File "/var/tmp/portage/net-p2p/elements-23.3.0/work/elements-elements-23.3.0/test/functional/test_framework/test_framework.py", line 132, in main
    self.run_test()
    ~~~~~~~~~~~~~^^
  File "/var/tmp/portage/net-p2p/elements-23.3.0/work/elements-elements-23.3.0/test/functional/mempool_unbroadcast.py", line 27, in run_test
    self.test_broadcast()
    ~~~~~~~~~~~~~~~~~~~^^
  File "/var/tmp/portage/net-p2p/elements-23.3.0/work/elements-elements-23.3.0/test/functional/mempool_unbroadcast.py", line 53, in test_broadcast
    rpc_tx_hsh = node.sendrawtransaction(txFS["hex"])
  File "/var/tmp/portage/net-p2p/elements-23.3.0/work/elements-elements-23.3.0/test/functional/test_framework/coverage.py", line 49, in __call__
    return_val = self.auth_service_proxy_instance.__call__(*args, **kwargs)
  File "/var/tmp/portage/net-p2p/elements-23.3.0/work/elements-elements-23.3.0/test/functional/test_framework/authproxy.py", line 144, in __call__
    raise JSONRPCException(response['error'], status)
test_framework.authproxy.JSONRPCException: txn-mempool-conflict (-26)

Expected behavior

All unit tests pass.

Actual behavior

mempool_unbroadcast.py test fails.

To reproduce

Run unit tests.

System information

Elements 23.3.0 on Gentoo Linux amd64, compiled with gcc-15 (Gentoo 15.0.1_pre20250418-r1 p55) 15.0.1 20250418 (prerelease), testing with Python 3.13.3, configured as:

./configure --prefix=/usr --build=x86_64-pc-linux-gnu --host=x86_64-pc-linux-gnu \
    --mandir=/usr/share/man --infodir=/usr/share/info --datadir=/usr/share \
    --sysconfdir=/etc --localstatedir=/var/lib --datarootdir=/usr/share \
    --disable-dependency-tracking --disable-silent-rules --disable-static \
    --docdir=/usr/share/doc/elements-23.3.0 --htmldir=/usr/share/doc/elements-23.3.0/html \
    --with-sysroot=/ --libdir=/usr/lib64 --disable-static --enable-wallet --without-sqlite \
    --with-bdb --disable-usdt --without-miniupnpc --without-natpmp --enable-tests \
    --disable-bench --disable-fuzz --disable-fuzz-binary --without-qrencode --disable-ccache \
    --enable-asm --disable-zmq --enable-man --disable-external-signer --with-utils \
    --enable-util-cli --enable-util-tx --enable-util-wallet --disable-util-util \
    --without-seccomp --without-libs --with-daemon --without-gui --without-qtdbus \
    --with-system-libsecp256k1

The same test passed on a second run with the same configuration, indicating that the issue is intermittent or non-deterministic.