Not sure about that. If you haven’t added them no reason why it would be any different it’s just using same protocols to find them as other apps will I this case MDNs and TCP ports are used . It’s just a list until you have added them it’d not tracking anything. Be interesting if you power one off to see if they both disappear at same time or one hangs about.
I do think there is some oddities with Roon and Chromecasts though as in previous versions of Roon for no reason it stops seeing them and often they would drop off or just one would not show or would take an age to be discovered. This has happened mostly when there has been a core update and it’s done it several times after Roon update. Google Home and all other apps still see them perfectly fine and can cast and control them but Roon just wont.
Also as I said I have seen this behaviour with Airplay as did another user this was not resolved so I think this.maybe related to that. It maybe certain scenarios trigger it but it should not be happening.