This happens because Essentials stores the base damage stat as a byte, which is a whole number that can only have a value in the range 0-255, so if you try to write something like 256, it will "wrap around" back to 0 in order to preserve that range. The edit that silverlime suggested lets you write any integer without having the compiler throw an error message, however it won't actually preserve it because of said wrap-around (but you still need his edit, so keep it anyways). What you need to do next is have Essentials store the base damage stat as something with a bigger range than a byte - possibly a half-word, which has the next largest range of 0-65535. Go to Compiler, under "def pbCompileMoves", and find this line:
You should get two search results, both pretty close to each other. Change both of them to the following:
Again, make sure to change BOTH lines that show up as search results. I changed the second character from C to S, which changes the base damage from a byte to a half-word (found that here:
https://apidock.com/ruby/Array/pack). That should prevent the wrap-around from occurring now.