pg_upgrade to 15 fails on Windows because of xml_is_well_formed()

When trying pg_upgrade to upgrade Postgres 14 to 15 on Windows 10 this fails with:

pg_restore: creating FUNCTION "public.xml_is_well_formed("text")"
pg_restore: while PROCESSING TOC:
pg_restore: from TOC entry 647; 1255 23216 FUNCTION xml_is_well_formed("text") postgres
pg_restore: error: could not execute query: ERROR: could not find function "xml_is_well_formed" in file "c:/Program Files/PostgreSQL/15/lib/pgxml.dll"
Command was: CREATE FUNCTION "public"."xml_is_well_formed"("text") RETURNS boolean
LANGUAGE "c" IMMUTABLE STRICT
AS '$libdir/pgxml', 'xml_is_well_formed';

The same error occurs when trying pg_dump/pg_restore, but the restore continues without further errors and the database is usable after that.

I don't understand why this functions is included in the dump in the first place.
A pristine 15 cluster already contains that function.

Any ideas?
Thomas