As of Unity 2022.2, UI Toolkit is the default backend for the inspector, before then the Editor was implemented using IMGUI.
Check your editor is being drawn with UI Toolkit by using the UI Toolkit Debugger (). Attempt to pick elements from the editor that contains your drawer, if you can select things beyond the root you are using UI Toolkit.
There are custom packages (Naughty Attributes for example) that override all editors for MonoBehaviour and ScriptableObject (or UnityEngine.Object) subtypes. If a package like this is present in the project you will have to resort to creating a UI Toolkit Editor for the components that draw this property.
If you are trying to find these Editors in your project you can search for [CustomEditor(typeof(Object), [CustomEditor(typeof(MonoBehaviour), [CustomEditor(typeof(ScriptableObject), until you find the overriding editor.
You mustn't have properly implemented the CreatePropertyGUI method.
UI Toolkit property drawers cannot be drawn inside of an IMGUI context. Create a UI Toolkit Editor for the components that draw this property.
Optionally create a IMGUI fallback for the property drawer. This will allow your field to be drawn in IMGUI contexts. As IMGUI drawers can be displayed inside UI Toolkit contexts, you could just write an IMGUI drawer, though this will be less performant.
Unity, huh, how?