Custom fields randomly stop working

Question

get_the_ID() is a post tag. It is meant to be used inside a Loop. Per a comment above you are using it outside the Loop.

No, it’s not inside Loop. The code is simply posted within another
div, which holds slides for a slideshow.

get_the_ID like most post tags depends on a global variable called $post. If that variable is not populated, or is set to the wrong post, you get odd results. I think that is why you think this works ‘randomly’.

A quirk of WordPress functioning is that the $post object is set well before the theme templates load, but only for some pages, which means for some pages some functions will work even when used incorrectly outside a Loop.

I think you probably want to be using get_queried_object to get the ID rather than depending on get_the_ID() but there are caveats with that too and the context you are using this in is still not clear.

$pobj = get_queried_object();
$id = $pobj->ID;

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