We tested on a similar rig (RX570 + RX5700, Windows 10 1709, AMD driver 19.9.1) and we didn't encounter any problems. Does PhoenixMiner 4.5c work on the same rig? What is the version of the drivers? Also, please try to run PhoenixMiner 4.6b only with -list (i.e. Phoenixminer.exe -list) and see if it shows the list of GPUs properly. A possible but unlikely reason - Windows defender may be blocking or even deleting PhoenixMiner.exe.
Win 10 1903, Driver 19.9.1. -list also the miner just closes with 4.6b and 4.5c:
PhoenixMiner.exe -list
Phoenix Miner 4.6b Windows/msvc - Release build
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No CUDA driver found
PhoenixMiner.exe -list
Phoenix Miner 4.5c Windows/msvc - Release build
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No CUDA driver found
Edit: I have a Windows Defender exception for the folder that the miner is in. It hasn't deleted anything from that folder in years.
Edit2: 4.5c was working fine in this rig with just the Polaris and 19.5.2
Edit3: Turns out Windows Defender was blocking the miner... Haven't seen it block something like that before....