Custom fields not saving/updating

I’m adding custom metaboxes to custom post types, following this tutorial:
https://shibashake.com/wordpress-theme/add-custom-post-type-columns

All of my custom fields save the data entered when creating/updating a post, EXCEPT two of them.

The two are named ‘mnt_sup’ and ‘mnt_supteren’.

mnt_sup saves the data when creating/updating a post, but the ther one won’t do it no matter what I do.

I made sure no blank lines exist in functions.php or any of the files I include in it.
I also tried to name them differently, for example ‘mnt_sup’ and ‘mnt_area’ -> no luck!

I’ve seen this bug is out for quite some time. What the heck is going on?!

I’m building a custom theme for a friend of mine and that’s why I create metaboxes, so he doesn’t have to go to the custom fields, select a key and add a value (that’s too techy for him…)

So… any ideas why it’s not saving just this? (as I said, it IS saving all of my other custom fields…)

Thanks ??