OP which wallet are you using? Because electrum can very well sign and verify messages signed from bech32 addresses. I think some of the other segwit wallets like samurai don't have a feature to sign message yet. I'm assuming since electrum can sign message then bitcoin core 0.16 also should be able to do that when its released.
Just signed one with electrum.
Code:
Message: Signed message Address: bc1qyjqygpulgd6y69ync6wvd256qjltszekd2pz0x Signature: HwNkc1x7GtforrqObazbOOure4XV5Jpx51Uf/dQ7hLQNEhkO+914HqUupdYJd3LH0eBtfENXBWRj3mb+HbQOoDw=
I know this, too. I have not yet tried to figure out what Electrum is doing; but its nonstandard. Core cannot verify those signatures (as I know for certain), and AFAIK that will not change with v0.16.