Is bc1q8tu9khgjhf2uv9ac546tcuh3my739gzhrw67nh an Electrum only address*? It's for sure not a valid Bitcoin address, Bitcoin Core doesn't accept it, and SegWit addresses have no consensus on signing a message.
*I read about this for the first time just today, but can't find back the link.
It's a valid bech32 Bitcoin address. The bech32 support was included in commit aa624b6 of Bitcoin Core so will be included in the next major release.
Edit: Just to clarify by support I mean it will recognise them as valid addresses and you will be able to send to them and verify messages, not that it will allow you to create a bech32 wallet or import the private keys.