I looked for one too, but couldn't find any except for the aforementioned Biostar. All the mATX B350s lack quite a few features i want, so I've sadly decided to go ATX with the X370. A shame. Perhaps something will be revealed for you at Computex in two weeks.
Do keep in mind that Ryzen is still relatively new and as far as I've seen from Youtubers is that manufacturers didn't really get all that much time to develop for Ryzen.
So I'd say give it some time and it will come around
Hm yeah I really hope Computex brings something new, since I already waited a few months for Ryzen to be released and now I am still waiting for good mATX boards...
Also Ryzen is relatively new yeah, but at this time they had nearly 3 months time and companies like ASUS or MSI should be able to fix BIOS issues and release new boards at the same time in my opinion.
But we will see ^^
I really don't know why aren't we seeing mATX X370 boards. Chipset doesn't pack insane connectivity. mATX would be perfect. We had X99 mATX and there was a load to pack on small board.