503s are an uncommon issue, however we do see them from time to time. In my experience, the most common reason behind an app encountering 503s is due to a throttle being applied.
API calls are subject to rate-limiting, which means that only a certain amount of requests can be processed per minute. If an app attempts to exceed the rate maximum, or starts to exhibit bot-like behaviour, the IP/app can have a temporary throttle applied and the requests will be blocked from the server, resulting in a 503.
I would strongly recommend approaching your app developer and have them review their call rate. If the issue is intermittent rather than constant, and the error is 503, then it's a strong likelihood that their issue is automated temporary throttles on their app. They should be able to resolve this by reducing the call rate where necessary.