Block caching issue when using a custom template instead of user_relationships-pending_block.tpl.php

I was trying to use a customized user_relationships-pending_block.tpl.php instead of the default one provided by the module user_relationship_blocks.
My changes were minimal, replacing (last line of the document):

print $output ? implode('', $output) : t('No Pending Requests');

by

print $output ? implode('', $output) : '';

in order to hide the block whenever there is no information to display (since content is empty, block is not displayed).

So I put the customized template in my theme directory, clear the theme cache and I was happy to see that my change was taken into consideration. But I noticed something weird, the block would not be updated immediately whenever changes were made to my "pending" relationships. To get the block updated, I had to clear the theme cache every time (which is problematic).

I am not certain where the issue is. If using the original template, the behaviour is correct and the block get updated as soon as I request or cancel a relationship (expected behaviour that match the code of the module: the cache is properly set to BLOCK_NO_CACHE). I have not tested with the other block template, but I suspect they will exhibit the same behaviour.

The issue is not related to my theme (I tested the issue with the Garland theme and the same problem occurs). I have resorted to modify (for the time being) the template provided by the module.