The ROCm compilers are slightly different, the newest GCN arch is probably not well tested for compute yet. The ROCm stack's development and testing effort strongly focuses on their 'pro/server' line cards. Is there a Navi WX or the equivalent yet?
Granted that I haven't really tried properly, as the card does show up as a compatible device, but trying to run Tensorflow on it fails due to associated GPU code not being available. Perhaps it doesn't have official support yet due to needing more compiler work.