issue with add_shortcode extacting $atts parameter

in my plugin I use add_shortcode to create a custom BBcode:

add_shortcode( 'pgn' , array(&$this, 'shortcode_pgn') );

function shortcode_pgn( $atts = array(), $content = NULL ) {
}

It used to work like a charm, extracting BBcode parameters using the $atts array. Recently, with no relevant changes in the plugin (so it must be a ww.wp.xz.cn update, now I’m using 3.4.2, but the bug migh have been unnoticed for a while) the plugin behaves incorrectly, it seems parameters are not captured correctly in $atts. Specifically, the value of the last supplied parameters is not read correctly and the values of the previous parameter is read instead.

For example, with

[pgn height=460 ss=40 h=320 eo=t hd=l am=n ih=e rm=1
pd=/data/file.pgn xxx=yyy]

the value $atts[‘xxx’] is not ‘yyy’ but ‘/data/file.pgn’

Has anyone seen the same issue?
Or any way to verify and fix the issue?

Thanks.