Instead of having to specify a specific default value, it would be nice to just have the answer pre-selected. I only have one option for each cascading select by the way...
We currently don't autopopulate a cascading select when it has only one value. If you have a situation where there is a 1:1 relationship between the filtering and cascaded values, it might make sense to support this via a pulldata() lookup system rather than a cascading select.
Based on what you've described, I wouldn't be using a dropdown/autocomplete question, as the selection actually happens on the previous question (if there is only one choice after filtering). Instead, I would use a text question using pulldata to look up the values - see Formulas—Survey123 for ArcGIS | ArcGIS for more information.
