I'm having the same issue. I've been working with the store I bought it from and it's got us all scratching our heads.
Ryzen 5 1600 + Gigabyte AB350 Gaming 3 mATX version.
Updated bios to F5 in the first instance because the newly built system would hang shortly after getting into a new Windows installation. It worked for a few boots then gave the same symptoms as above. Between myself and the store we have tested with:
5+ GPUs
5+ PSUs
new mobo (of the same model)
Ryzen 3 <--- it posts with this
Ryzen 7 <--- it posts with this as well
3+ types of RAM, all different slots/combinations etc
The Ryzen 5 was put in another mobo and it posts.
One frustrating peculiarity: After testing with the Ryzen 7 at the store, my CPU was put back in and the whole thing worked. I brought it home, installed the OS and stress tested/benchmarked for an hour or so. All was well so I fired up a game and got to it. Everything was fine, after finishing my game I went off to do something else to return and find a black-screen. Back to square one, no post. Re-testing at the store and with my own hardware ensued and still no go, it will just not play nice with this mobo.
The store has spent so much time on this (as have I, and I'm no slouch) they are giving up. Their solution is to sell me a Ryzen 7, which I'm not doing.
I need some help with this one Gigabyte, I'm not giving up till I have this working.
Looking forward to some input.
Thanks