Sometimes there is just issues with the reporting of certain streams once in a while. Unless it’s a persistent issue for prolonged periods of time there’s not much you can do.
Also polling the streams endpoint every 30 seconds is somewhat excessive as all Twitch API endpoints use caching, and if you poll too frequently you can even run into a circumstance where you hit 1 cache server on one request, then on the next you hit a stale cache server that shows different (and older) data. It’s generally recommended to not make the same requests more than once every minute at most.