Seems the way AWB encodes urls results in a non working search query
https://en.wikipedia.org/w/api.php?action=query&list=search&srwhat=text&srlimit=max&srsearch=insource:/\%3Ccenter\%3E\%3Cgallery/i vs https://en.wikipedia.org/w/api.php?action=query&list=search&srwhat=text&srsearch=%27insource%3a%2f%5c%3Ccenter%5c%3E%5c%3Cgallery%2fi%27&srlimit=max
Could be an html encoding bug like T91219 ?