But unfortunately pretty routine. In two out of two cases where I've claimed compensation for delayed/cancelled flights under the EU law, the airlines have outright lied about the causes and denied compensation.

In both cases, they were forced to pay out in arbitration after the arbitor determined that their stated causes were false (e.g. providing the tail number for a different aircraft that was legitimately delayed) , but there was no penalty for lying.

In such an environment, it's quite rational for them to lie and deny.