I am seeing the same issue. I have Alexis IO4, Studio One 5 running on a MAC.
In Garage Band the IO4 inputs are correctly routed and show as Ch or I/P 1,2,3,4. I can map each input on the IO4 correctly into Garage Band.
On Studio One the inputs show up as Surround Sound, like Front Left, Front Right, Rear Left and Rear Right. Only 1 and 2 work.
If I try and map input 3 (eg displayed as Rear Left) then it just goes blank after selecting it.
IO4 is my Input and Output device and Ch 1 and 2 are fine, its just I cannot map or use Ch 3 or 4.
MIC Security permissions are correct.
MAC Audio MIDI also shows the Surround Sound labels, but Garage Band manages to map them OK. What am I missing here?