After the update to Jenkins 2.249.1 parameters that include "object" within the properties have stopped working, example of this:
schema: {
            "type": "object",
            "title": "Build_Parameters",
            "properties": {
            "ThisIsNotSHOWN": {                "type": "object",
                "properties": {
                    "Opt1": 

{                         "type": "boolean",                         "propertyOrder" : 1                     }

,
                    "Opt2": 

{                         "type": "boolean",                         "propertyOrder" : 2                     }

                },
            "ThisIsShown": 

{                  "type": "string",                  "propertyOrder" : 2             }

another example can be found in the documentation: "Advanced example"
https://wiki.jenkins.io/display/JENKINS/Extended%20Choice%20Parameter%20plugin
Both are simple not shown in the 'build with parameters page'