The ArcGIS Survey123 web app only displays the first 512 choices listed in the choices sheet for a select_one question with the autocomplete appearance.
The limitation of the number of choices is a browser limitation for Chrome or Chrome-derived browsers such as Microsoft Edge. Refer to https://bugs.chromium.org/p/chromium/issues/detail?id=1026739&can=2&q=datalist for more information. This is mentioned in the ArcGIS Survey123 documentation: https://doc.arcgis.com/en/survey123/browser/get-answers/knownissuesgetanswers.htm#ESRI_QUESTIONANSWER_2A44F5C3B19B49C9828A6BFB41AD6CBF.