If you start Roon core and some other player (StylusEP or HQPlayer) you will notice that your DAC is no longer visible in Roon Client interface (as opposed to when Roon core is started alone).

This is expected!

When a player accesses the audio device, this access is exclusive! No other player can play on this device until the first player relinquishes its access. When Roon core starts with some other player, Euphony ensures that this player will get exclusive access first. Otherwise, there is no point in starting other player since it will not have audio device access - Roon gets it first.
It is very unfortunate that Roon will not show the device at all if it cannot obtain exclusive access. Roon should show it anyway. It could mark it as disabled or just show it and at least report an error about the device being busy (or something like that) on playback attempt.

Admittedly, in an ideal situation it could all work seamlessly if there was tighter integration between Roon and other players - a mechanism for Roon to relinquish its own access and signal the other player to acquire it or vice versa - signaling to other players to release access so Roon can acquire it but this must be done in tight cooperation with Roon.

In conclusion, when you select Roon + other player expect not to see the DAC in Roon interface - this is normal behavior.

In general - you should only rely on Euphony to show all your output devices - regardless if it can have exclusive access or not.