This is by design. If an ARAnchor is disabled then it is removed from the ARAnchorManager. If the `destroyOnRemoval` property of the ARAnchor is true then the GameObject of that trackable will be destroyed. In this case, user will need to set that property to false if they want to keep the game object after disabling it.
https://docs.unity3d.com/Packages/com.unity.xr.arfoundation@5.0/api/UnityEngine.XR.ARFoundation.ARTrackable-2.html#UnityEngine_XR_ARFoundation_ARTrackable_2_destroyOnRemoval