update_post_meta() not saving data inside of save-post filter

I solved my own problem.

save-post hooks in before the form is saved. It even gives us access to the $_POST object and allows us to interact with it before the original save happens.

What this means essentially is that I’m overwriting my values with blank formdata.

Rather than updating post meta, I’ve documented and am writing to the $_POST object the updated values, which are then submitted as part of the original save post.

$hat = update_post_meta( $post_id, 'field_name', $update_var );

Becomes

/**document  this well or you'll drive someone insane in the future.**/ 


$_POST['field_name'] = $update_var;

Thanks everyone for your thoughts and attempts to help. You no doubt inspired me to think about it.