Multiple choice lookup field does not render on edit form

I have, for the last few years, used a field called “Enabled” and a calculated field which returns the value of the Title field if Enabled is true. If Enabled is false, I return an empty string (“”). This is the formula: =IF([Enabled],[Title],””)

This allows the administrators of the site to temporarily enable/disable options from displaying in the new and edit form of a list which references those items in a multiple-choice lookup column with out-of-the-box functionality instead of using code. This has worked for many years, but all of a sudden, after the user-facing list (not the lookup list) has had thousands and thousands of items, this is causing issues.

On the display form, the disabled items in a lookup field used to not appear while the enabled items would still show. Now, the disabled items are displayed as numbers showing the corresponding lookup ID.

On the edit form, if a user had previously selected an item in a lookup and that item has now been disabled (blank), the item would fall off the list of selected results and not be available in the list of selections. Now, the entire selection candidate and results for the lookup field are not rendered. I inspected the HTML for the form, and there is literally nothing on that row other than the initial table cell with the field name.

This type of functionality (blanking out a field used in a lookup selection in order to hide it) still works on other lists and other sites. How can I narrow down what it suddenly causing this issue on this one list?