If you have a case that labels are lost for ArcGIS Server services, please check if this is for a given folder (s). We have observed a few case that calls to ArcGIS Server admin API to list services in a given folder sometimes don't return expected response or time out. In the current release, ArcGIS Monitor would see it as deleted folder services and effectively unregister those services. These services would be re-registered the next time the folder info is collected successfully, and the proper response returned. This would explain why labels "disappear". Because this would be a brand-new registration and naturally it would not have original labels.
We'll add extra logging to capture this case better and considering other improvements, e.g. retries. But this might not address the cases where the ArcGIS Server admin API does not return expected response repeatedly. So, if you have this case, you may want to verify it using admin api and check logs for the underlying issue. If you determine this is ArcGIS Server related, you may want to contact Esri Tech support to triage this case.
In the meantime, I would be very much interested to learn the workflows these labels support and potentially offer viable alternatives.
