This issue was discovered through an ongoing maintenance effort on dewiki and seems to have started only with beta14, otherwise it would likely have been reported previously.
Typical example:
''Ellen Dennhoven in Westdeutsche Zeitung, Mönchengladbach 18. November 2006 [http://www.wz-newsline.de/sro.php?redid=134956]{{Toter Link|date=2018-04 |archivebot=2018-04-06 13:27:14 InternetArchiveBot |url=http://www.wz-newsline.de/sro.php?redid=134956 }}''is converted to
''Ellen Dennhoven in Westdeutsche Zeitung, Mönchengladbach 18. November 2006 [{{Webarchiv|url=http://www.wz-newsline.de/sro.php?redid=134956 |wayback=20070926235513 |text=Archivierte Kopie |archiv-bot=2019-04-06 10:09:46 InternetArchiveBot }}''which leaves a lonely, visible [ in front of the {{Webarchiv}} template.
This seems to happen only for cases where the URL contains a query string (in the example: ?redid=124956).
Edits which where reported: