-snip- BTW, I found that Bitcoin Core cannot sign using native segwit addresses (bc1...). Somebody should implement that. -snip-
You cannot sign a message using the SegWit address. I've also tried it.-snip-
I have also asked about this issue. Here's the answer from achow101:
There is no solution and you cannot sign a message with a segwit address. This has been the case since segwit was introduced. See https://github.com/bitcoin/bitcoin/issues/10542