Adrenaline seems to run its calculations each time it attacks, and determines whether or not to continue based on its attack and how many attacks it has already made. In the case with the wyrm, a creature with 6 attack should attack 3 times, and it has attacked once, so it will attack again. On the second attack, its attack is now 3, so it should get four attacks and has attacked twice, so it will attack again. On the third attack, its attack is now 3, so it should get four attacks and has attacked three times, so it will attack again. In the case with the dragon, on its first attack it has 30 attack, so it should only get one attack, and it has attacked once, so it does not attack again.
In other words, adrenaline does not notice that the attack has been reduced until the second attack happens, and creatures with 30 attack do not get a second attack.
Similarly, when you combine acceleration and adrenaline, the final attack will sometimes be made with a higher attack score than normal to receive that many attacks, because the decision was based on how much attack it had on the previous attack.