Function for Sticky Post on Categories causes issues with 404 behavior

by

The solution was to replace:

if (    !is_admin() // IMPORTANT, make sure to target front end only
     && $q->is_main_query() // IMPORTANT, make sure we only target the main query
     && $q->is_category() // Only target category archives
) {

for:

if (    !is_admin() // IMPORTANT, make sure to target front end only
     && $q->is_main_query() // IMPORTANT, make sure we only target the main query
     && $q->is_category() // Only target category archives         
     && $q->is_tax(get_query_var ( 'category_name')) // Only target existing category names 
){