Object rotation snaps back to 0°

is that the complete code that manipulates the transform in question?

You definitely have a problem somewhere else in your code, please edit and post your entire script

     [SerializeField]
     private GameObject placeableObjectPrefab;
 
     [SerializeField]
     private KeyCode newObjectHotkey = KeyCode.A;
 
     private GameObject currentPlaceableObject;
     private float mouseWheelRotation;
 
     private void Update()
     {
         HandleNewObjectHotkey();
 
         if(currentPlaceableObject != null)
         {
             MoveCurrentPlaceableObjectToMouse();
             MainRotation();
             ReleaseIfClicked();
         }
     }
 
     private void ReleaseIfClicked()
     {
         if(Input.GetMouseButtonDown(0))
         {
             currentPlaceableObject = null;
         }
     }
 
     private void MainRotation()
     {
         if (Input.GetKeyDown ("e"))
         {
             currentPlaceableObject.transform.Rotate(transform.rotation.x, transform.rotation.eulerAngles.y + 90, transform.rotation.z);
         }
         else if (Input.GetKeyDown ("q"))
         {
             currentPlaceableObject.transform.Rotate(transform.rotation.x, transform.rotation.eulerAngles.y - 90, transform.rotation.z);
         }
     }
 
     private void MoveCurrentPlaceableObjectToMouse()
     {
         Ray ray = Camera.main.ScreenPointToRay(Input.mousePosition);
 
         RaycastHit hitInfo;
 
         if(Physics.Raycast(ray, out hitInfo))
         {
             currentPlaceableObject.transform.position = hitInfo.point;
             currentPlaceableObject.transform.rotation = Quaternion.FromToRotation(Vector3.up, hitInfo.normal);
         }
     }
 
     private void HandleNewObjectHotkey()
     {
         if(Input.GetKeyDown(newObjectHotkey))
         {
             if(currentPlaceableObject == null)
             {
                 currentPlaceableObject = Instantiate(placeableObjectPrefab);
             } else {
                 Destroy(currentPlaceableObject);
             }
         }
     }