It seems REST API in 10.5 deprecated simple sintax on layerDefs:
ArcGIS REST API
Simple syntax is not supported as an expected value for layerDefs parameter starting 10.5.
This is pretty hidden in the documentation by the way...
As a side effect, it seems imageparameter.layerDefinitions no longer works since it still translates requests to the simple sintax:
ImageParameters | API Reference | ArcGIS API for JavaScript 3.19
In our server we get an error when debugging http requests on the browser's console:
{"error":{"code":400,"message":"Invalid 'layerDefs' is specified","details":[]}}
Our code is pretty simple and worked last week against a 10.3 server.
Can anyone reproduce and confirm this?
EDIT: Just to add that the simplest sample from esri does not work against a 10.5 server:
Layer definitions on a dynamic map service | ArcGIS API for JavaScript 3.19
It seems a bug to me... I wish I'm proven wrong...
Thanks.