Currently you're not able to run Flows that contain a "on demand" for selected item trigger in a new Power Automate environment against a SPO list. You can only run this if the Flow is created in the default environment.
I've created a new Flow environment call UAT
Created the workflows in this environment
Set all the connections to point to the SharePoint list
When I go to the list, I can't see the Flow that is in the Flow UAT environment
When I hover over Power Automate I can't see my Flow there
So I think it would be good if you can choose which environment you want the SPO list to look at.
Also, I created a button that runs the Flow when a user clicks it. However, because the Flow is now on a new environment this button no longer works as the SPO list can only reference flows in the Default environment. I think it would be a good idea to somehow reference workflows on new environments in JSON.