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Electrum includes a locktime in all transactions by default.
Usually the value is equal to the current block height, eg 592918, however it sometimes drops perhaps 20 or so blocks.
What is going on here?
The locktime value is deliberately set to discourage a subtle attack known as "Fee sniping" and randomly set to an earlier block height to improve privacy (eg for CoinJoin users that need more setup time).
This is also done in Bitcoin Core with the nLockTime.
Required, but never shown