Index: wp-includes/taxonomy.php
IDEA additional info:
Subsystem: com.intellij.openapi.diff.impl.patch.CharsetEP
<+>UTF-8
===================================================================
--- wp-includes/taxonomy.php	(revision 9c75ea591c3f3a7c05a8ea7ff3a6fa2911d02305)
+++ wp-includes/taxonomy.php	(date 1584025983751)
@@ -338,7 +338,7 @@
  *
  * @global array $wp_taxonomies Registered taxonomies.
  *
- * @param string       $taxonomy    Taxonomy key, must not exceed 32 characters.
+ * @param string       $taxonomy    Taxonomy key, must not exceed 32 characters, lower case.
  * @param array|string $object_type Object type or array of object types with which the taxonomy should be associated.
  * @param array|string $args        {
  *     Optional. Array or query string of arguments for registering a taxonomy.
@@ -414,6 +414,10 @@
 function register_taxonomy( $taxonomy, $object_type, $args = array() ) {
 	global $wp_taxonomies;
 
+	if ( ! empty( $taxonomy ) ) {
+		$taxonomy = strtolower( $taxonomy );
+	}
+
 	if ( ! is_array( $wp_taxonomies ) ) {
 		$wp_taxonomies = array();
 	}