did you configured your YaCy peer to run in "Search Portal" mode (in /ConfigBasic.html "Search portal for your own web pages")? Because the yacysearch.json (or yacysearch.html) API is performing realtime P2P search, its results are not completely deterministic in P2P mode, and as you report it there is maybe something more to fix.
By the way, if you only need results from your own index, I would recommand you using the /solr/select API which is faster and more reliable for this use. This approach is now used as default
to demonstrate the YaCy Bootstrap Webclient usage.