Incongruent
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I've read the water knot can back off when not under load as well.
According to Wikipedia, it can.
"Once tied, for additional security each end should be tied in a double overhand stopper knot around the other standing end.
Some testing has shown that the water knot, in certain conditions, can slip very slightly but very consistently, with cyclic loading & unloading at relatively low forces; it is the tail on the exterior that slips (this would be the blue tail in the image presented here). In tests using 9/16 in (14.3 mm) tubular nylon webbing, repeated loading and unloading with 250 lbs (113 kg) caused one of the 3 in (76 mm) tails to work back into the knot in just over 800 loading cycles. Another test showed similar results for Spectra tape (but not for new, 1 inch tubular nylon). And yet the knot can be loaded to rupture without slippage. These results validate the need to leave adequate tails and inspect water knots before each use. With single overhand knot safeties on either end, the combination eventually seized and the slipping stopped.
Although used extensively in climbing and caving, there is some opinion that the water knot is unsafe. According to Walter Siebert, several deaths have been reported due to failure of this knot. In Germany, the knot is sometimes called todesknoten, which means death knot."
The bolded text is a possible remedy. The tails should be long enough.