that is insane, it looks like pseudocode

I like your solution, but hate the readability of your code. Your variable names are not intuitive and make your code confusing to read.
Consider changing "buffer" to "placeValue" or similar, "expected" to "result" or "answer" or similar, and "appendDig" to "digit" or "currentDigit" or similar.

what does reduce(1,(a,b)) mean?

This fails for an empty array by returning 0.

also available is Character::getNumericValue

Collectors.joining()
also how do you get stream indentation to line up when posting to kata

i keep forgetting to use .chars() ty

imho it's always a good idea to be prepared
(you will often see here on codewars, that the visible test-cases not always cover all posible incidents)