We have been working with dynamiclayer functionality in ArcGIS Server services for years. This makes it possible to customize symbology and labeling on existing mapservices (posting dynamiclayer as JSON).
Since our customers started using ArcGIS Pro to publish services - the labeling part of the dynamiclayer does not work. No labels show on the map - regardless of the labelinginfo object. The extact same calls work fine when published with ArcMAP.
It has been tested not to work on different AGS versions (10.7, 10.9.1 and 11), but only when published with ArcGIS Pro.
It seems to be a bug?