I recently had something similar although a little different. Still might help you though.
I had a rig running 6 RX5700 GPUs happily mining away on Claymore’s using DaggerHashimoto. I decided to move them to another rig and then installed 6 RX570s on the original rig. After all my tweaking and setup, I ran Nicehash and it benchmarked okay. When it started up however, Claymore’s would identify 6 GPUs but only mine on four of them - and the four it mined on kept changing everytime I restarted Nicehash. In the Nicehash mining rig manager it would tell me that all 6 GPUs were mining but that the hashrate of 2 of them was zero.
I ran Claymore’s outside of Nicehash mining Eth to a pool, and it used all 6 GPUs successfully. I tried everything but in the end I had to do a clean uninstall of Nicehash and re-install it all again.
This is where it might help you. After I installed Nicehash, I kept getting an error on Claymore’s benchmarking for all 6 GPUs. So I went to the Plugins tab and uninstalled Claymore’s, waited a little while (for some reason it wouldn’t re-install immediately) and once it was back up and running, I got perfect benchmarking and smooth running ever since.
Might not hurt if you uninstall/re-install Pheonix and see if that makes a difference to the 2 GPU’s being able to successfully benchmark.