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If a Tyranid unit fails their instinctive behaviour test at the start of their movement phase, do they negate the rule altogether if a synapse creature moves into range? The codex says synapse creatures allows other units to ignore instincitve behaviour, does this kick in even after the units have taken their instinctive behaviour tests?

iirc no. They have to be in range at the start of your turn. The test is taken like a falling back test, which happens before the movement phase/ the very begining of it.

Synapse creatures negate the need for a IB test, if there is no creature in range and you take the test, you are subject to the effects even if you move a Synapse creature into range after the fact.