Hm, we have had similar issues before, one example:
https://tracker.mender.io/browse/MEN-2428
Above is fixed though, and not sure if it is related.
There was also another thread on this topic, Mender-proxy/deployments unable to upload files to minio. This was related to on-premise though, and should be resolved in hosted Mender.
In conclusion, there has been issues with very large Artifacts, where the API gateway timeouts.
@tranchitella probably has the most up to date information here.