Nominatim & osmnx: Looking for cities Aarau & Fribourg, getting "City Märt" in Aarau and Freiburg City Hotel in Freibug im Breigsau

I am coding in Python, using the package osmnx, and in particular the function geocode_to_gdf from osmnx to get the geometry of some cities in Switzerland. If I send the request "Fribourg, Switzerland", I get the canton as first answer. If I send the request "Aarau, Switzerland", the risk is to get the "Bezirk"/district. To avoid these problems, I send the request as a dictionary: "{'city': 'Aarau, Schweiz'}" or "{'city': 'Fribourg'}". Seems safer and is recommended by the author of the osmnx package on Stackoverflow. I have two questions:

• Why are the results not stable? Depending when I run it and how (in code or in a jupyter notebook), I get different results. "{'city': 'Fribourg'}" sometimes gives as output the city of Fribourg in Switzerland and sometimes "Freiburg City Süd Hotel, 19, Heinrich-von-Stephan-Straße, Unterwiehre-Nord, Wiehre, Freiburg im Breisgau, Baden-Württemberg, 79100, Germany". "{'city': 'Aarau, Schweiz'}" sometimes gives "City Märit, Aarau, Bezirk Aarau, Aargau, 5000, Switzerland".
• Why are the results sometimes empty? "{'city': 'Fribourg, Switzerland'}" generates 0 answers. Same with "{'city': 'Fribourg, Schweiz'}", as well as "{'city': 'Fribourg, Suisse'}"

I'm not sure if it is more a question about the data structure in OSM, or a question regarding the package osmnx.