Custom taxonomy slug bug

[WebCimes]

I have a problem in some case when I use custom_taxonomy_slug,

For exemple when I try to do :

http://xxx.com/opportunities/%cat_opportunities%-%postname%

The single file "single-opportunities.php" doesn't work, but If I do this :

http://xxx.com/opportunities/%cat_opportunities%--%postname%
http://xxx.com/opportunities/%cat_opportunities%/%postname%
http://xxx.com/opportunities/%cat_opportunities%-test-%postname%

The single file "single-opportunities.php" work in this case, so I doesn't understand why a single "-" cause a bug ?

Thanks