If you have your own API deployed on your server go to the file (https://js.arcgis.com/3.18/esri/dijit/editing/Editor.js) and find:
function(a){return"esriGeometryPolygon"===a.geometryType&&a.visible&&a._isMapAtVisibleScale()}
To fix the bug with merging the above must be replaced with:
function(a){return a.geometryType&&a.visible&&a._isMapAtVisibleScale()}
For some reason Esri is checking if the geometry type is polygon before UNION function is used on the geometry server? WHY???
Removed "esriGeometryPolygon"===a.geometryType
Everything working as expected.
Hope this helps.
Regards,
Andrzej