Old submatches returned by match-string et al

There seems to be a case where stale values are returned by match-beginning, match-end and match-string. To reproduce: 1. Start XEmacs with -q -no-site-file. 2. Make an empty buffer, enter "ab" and go back to the beginning. 3. Evaluate (re-search-forward "\\(a\\)"). 4. Evaluate (re-search-forward "b"). Now (match-string 0) will return "b" as expected, but (match-string 1) will return "a" from the previous search. From the documentation I expect it to return nil (which is also how it works in FSF Emacs). `match-beginning' and `match-end' behave similarly. I've verified this in XEmacs 21.4 (patch 6).