I have a rule defined to drop rocks from stone when broken with a crude pickaxe. On the final harvest of the tool's life, when the tool's durability reads 0/32, the block drops cobblestone instead of rocks.

A possible cause may be that the tool is breaking and by the time the harvest event is fired, the tool doesn't exist in the player's hand anymore, thus not matching the appropriate rule.

This needs to be reproduced in a minimal environment to ensure that it isn't a conflict with another mod.