The fix is created by Bernhard Übelacker from apple/cups #5826.

The problem is described in detail here. I just removed:

--- a/backend/ipp.c
+++ b/backend/ipp.c
@@ -3026,6 +3026,7 @@ report_attr(ipp_attribute_t *attr)        /* I - Attribute */
           valptr += strlen(valptr);
           break;
 
+      case IPP_TAG_STRING :
       default :
          /*
          * Unsupported value type...

from the original patch - it looks okay without it (since the attribute is found as IPP_TAG_STRING and the tag doesn't have a case in switch, it will fallback into default: case automatically).

Regarding PWG standard IPP_TAG_STRING is the correct one - printer-alert is defined as octet-string (meaning ippAddOctetString must be used) and IPP_TAG_TEXT tag works with ippAddString, so based on this it looks like IPP_TAG_STRING is the correct one.