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Describe the Bug:
Vampire Nocturnus' ability doesn't trigger immediately, only once combat beginns. It triggers during combat if the top card changes, though (i.e. when opponent is attacked by Nemesis of Reason).

Which card behaved improperly?
Vampire Nocturnus

Which update are you using? (date, name)Which type? (duel, gauntlet, sealed deck)
Test Manalink 3.0 Build as of 01-Aug-2019 updated till Nullstone Gargoyle in commit 01e3541. Duel

What exactly should be the correct behavior/interaction?
Vampire Nocturnus' ability should trigger immediately when it comes into play or when the top library card changes.

Are any other cards possibly affected by this bug?
No.

Although it doesn't change visually to correct p/t immediately did you see it not dealing the correct damage according to the p/t it should have? I confirm the visual issue but in my tests did the correct damage and could suffer the correct amount of damage according to top card of the library changing it's p/t.

Damage dealing worked correct. Once combat begins power and toughness are updated properly.