You also need to provide fields you want to see for a returned entity. Here is the list of fields for StateBundleElement. For example, the request GET api/issues/DEMO-1/activities?categories=CustomFieldCategory&fields=timestamp,author(login),added(name),removed(name),category returns
[
{
"removed": [
{
"name": "Submitted",
"$type": "StateBundleElement"
}
],
"added": [
{
"name": "Open",
"$type": "StateBundleElement"
}
],
"timestamp": 1577923200000,
"category": {
"$type": "ActivityCategory"
},
"author": {
"login": "root",
"$type": "User"
},
"$type": "CustomFieldActivityItem"
},
{
"removed": null,
"added": 1592654400000,
"timestamp": 1593356400000,
"category": {
"$type": "ActivityCategory"
},
"author": {
"login": "Demo.User",
"$type": "User"
},
"$type": "CustomFieldActivityItem"
},
{
"removed": 1592654400000,
"added": 1592740800000,
"timestamp": 1593445767000,
"category": {
"$type": "ActivityCategory"
},
"author": {
"login": "Demo.User",
"$type": "User"
},
"$type": "CustomFieldActivityItem"
},
{
"removed": 1592654400000,
"added": null,
"timestamp": 1593445767000,
"category": {
"$type": "ActivityCategory"
},
"author": {
"login": "Demo.User",
"$type": "User"
},
"$type": "CustomFieldActivityItem"
},
{
"removed": [
{
"name": "Open",
"$type": "StateBundleElement"
}
],
"added": [
{
"name": "In Progress",
"$type": "StateBundleElement"
}
],
"timestamp": 1703002756610,
"category": {
"$type": "ActivityCategory"
},
"author": {
"login": "root",
"$type": "User"
},
"$type": "CustomFieldActivityItem"
}
]