I've always vs I always

The first example "I knew it was you because you always knock first, and then enter", is more correct.

The second one "I knew it was you because you've always knocked first, and then entered', is grammatically OK, but may not be suitable to talk about one's nature.

second one is not grammatically correct because it Doesn't have perfect tense blending. Simple past tense doesn’t match with present perfect.

When someone does something regularly use simple present. Don't make a big deal out of it