If you used this method after plugin uninstall your url hit this action, Did you check this case ?
Did anyone run into this?
I am facing the same issue
Try to use this method "pluginFinder.GetPluginDescriptorBySystemName()" in RouteProvider.cs
Using this method you will get plugin object which you can be use in IF BLOCK under which you can write override route
When plugin isn't installed. you will get null object.