Problem with clear_cache and cache groups

Hello,
I'm using smarty 2.6.18, I've just upgraded from smarty 2.5 (...).
I'm using clear_cache to clear cache groups, it did work with smarty 2.5
but now it doesn't work anymore. Smarty's config and file permissions
should be ok because clear_all_caches works correctly, but if I do a
clear_cache("group1") it doesn't delete the cache for the page I
displayed using $tpl->display("mytpl", "group1|group2");
The call to clear_cache returns true... but the files are still in their
directories.
I'm using php 5.2.2 on gentoo linux with apache2.
How can I debug (and fix) this??

-- 
Francesco