bitcoin: "Fee too high" CI error in wallet_send.py

wallet_send.py --legacy-wallet line 332

https://cirrus-ci.com/task/6260771733635072

2022-01-25T13:05:06.243000Z TestFramework (INFO): Test setting explicit fee rate
2022-01-25T13:05:06.666000Z TestFramework (ERROR): Assertion failed
Traceback (most recent call last):
  File "/tmp/cirrus-ci-build/ci/scratch/build/bitcoin-x86_64-pc-linux-gnu/test/functional/test_framework/test_framework.py", line 132, in main
    self.run_test()
  File "/tmp/cirrus-ci-build/ci/scratch/build/bitcoin-x86_64-pc-linux-gnu/test/functional/wallet_send.py", line 332, in run_test
    assert_fee_amount(fee, Decimal(len(res["hex"]) / 2), Decimal("0.00004531"))
  File "/tmp/cirrus-ci-build/ci/scratch/build/bitcoin-x86_64-pc-linux-gnu/test/functional/test_framework/util.py", line 45, in assert_fee_amount
    raise AssertionError("Fee of %s BTC too high! (Should be %s BTC)" % (str(fee), str(target_fee)))
AssertionError: Fee of 0.00000993 BTC too high! (Should be 0.00000983 BTC)

Haven’t seen it fail yet locally on the same branch while running:

(for i in {1..1000}; do test/functional/wallet_send.py --legacy-wallet --pdbonfailure ; done)

This test assertion was added in October 2020 (603c0050837) and the get_fee() test utility helper in October 2021 (80dc829be7f), so if the error is new then it might stem from a more recent change.