Method reference dropping from OnClick()

Problem: A method referenced by one of my buttons randomly comes up as "Missing":

Details: The referenced method is not static, although that shouldn't be an issue in 5. Here's the referenced script:

 using UnityEngine;
 using System.Collections;
 
 public class MenuManager : MonoBehaviour
 {
     // …
     //Referenced method
     public void ShowMenu(Menu menu)
     {
         if(menu.IsPopup == false)
         {
             //Closes the current menu
             if(CurrentMenu != null)
             {
                 CurrentMenu.IsOpen = false;
                 CurrentMenu.IsActive = false;
             }
             
             //Sets new menu (target of function) to CurrentMenu, and opens current menu
             CurrentMenu = menu;
             CurrentMenu.IsOpen = true;
             CurrentMenu.IsActive = true;
         }
         else
         {
             if(CurrentMenu != null)
             {
                 CurrentMenu.IsOpen = false;
                 CurrentMenu.IsActive = true;
             }
             
             CurrentMenu = menu;
             CurrentMenu.IsOpen = true;
             CurrentMenu.IsActive = true;
             
         }
     }
 }

The object that OnClick is referencing has the component script on it, and the method in the component shows up in the OnClick drop-down menu in editor:

Attempted Solutions: I'll replace the missing reference by reselecting it from the drop-down menu. Then I save and close Unity, reopen it a few times or work on other scripts, and the method shows up as Missing again. I've tried recreating the button wholesale, renaming the script, moving the script to a difference object in the Hierarchy and referencing it there-- all to no avail.

Desperately seeking help. Thanks for looking.