This ruling has been removed from the official site. Preserved here on DiGiCA-NAVI for reference.
After declaring an attack with this Digimon, this Digimon's digivolution cards were trashed by an effect during the reaction timing window. Is the attack canceled?
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Official Ruling
No, once the attack is declared, the attack will continue even if you lose digivolution cards in the meantime.
Source: EX3-060 ExTyrannomon