My guess is that you are going back to edit the workflow after it has been saved. You delete a preset and then try to add a new one with the name of the deleted preset.
FFCore will not let you use the name of the deleted preset until after you save the workflow.
When you edit the workflow, we keep the original workflow until you save the updated workflow (this allows you to exit the workflow edting without saving changes). Since the original workflow is still in the system you cannot use the Route preset name in a new preset.
So, use a different name, save the workflow (which discards the original workflow) and then edit the workflow and rename the route preset. I tried this with v22.214.171.124 and it is working as expected with the above caveat.
I think I understand why that's done but I will ask development about it and see if there is anything that can be done.
We just release SP2 not too long ago so if there is opportunity to do anything it will be on the next service pack.
If i edit or create a new route, under "the name to use for this preset" seems to remember every route name ever used and no longer allows a given name to be saved as a preset if that name has previously been used, even if that route is not used in any workflow.
How can these old names no longer used in a workflow be removed.