From official "Nginx by HTTP" template, one of the triggers uses the "find" function, which does not target a time frame:

find(/Nginx by HTTP/nginx.get_stub_status,,"iregexp","HTTP\\/[\\d.]+\\s+200")=0 or nodata(/Nginx by HTTP/nginx.get_stub_status,30m)=1

This keeps all the item's values in the value cache. The item will be listed as the leader behind:

zabbix_server -R diaginfo=valuecache

And filling up zabbix_server.conf:ValueCacheSize.

I think the fix is to target a time frame like "5m" or "15m":

find(/Nginx by HTTP/nginx.get_stub_status,15m,"iregexp","HTTP\\/[\\d.]+\\s+200")=0 or nodata(/Nginx by HTTP/nginx.get_stub_status,30m)=1