Hi,
Commit 819409 introduced a change where the text-properties of tags
collected from a buffer are no longer stripped, which means tags may be
fontified in unwanted ways in completing-read (this became especially ugly
using org-modern and code for right aligning tags with display properties).

Something like this would fix it:
@@ -11974,7 +11974,7 @@ (defun org-get-buffer-tags ()
         (org-element-cache-map
          (lambda (el)
            (dolist (tag (org-element-property :tags el))
-             (puthash (list tag) t hashed))))
+             (puthash (list (substring-no-properties tag)) t hashed))))

Best,
Anders Johansson