This is one of the older search services that is directly powered by TomTom. If memory serves me correctly TomTom modified the backend of this service a year or two ago so that only the first 100 POI's may be retrieved. I believe there were concerns with individuals scraping the POI data and creating databases. The pagination is still there as many users will retrieve 10 results at a time and allow users to page to the next 10 results.
I'm not on the Azure Maps team, but wrote a lot of the code samples for Azure Maps and recall one where I had it paging through POI's and a few years ago it stopped getting more than 100 results. Testing the TomTom service that powers this Azure Maps API I see the same behavior: https://developer.tomtom.com/search-api/documentation/search-service/nearby-search