Fairly easy to reproduce
When taking the node that Shield bash gets a second charge, The first charge works as expected and clearly gets the recovery bonus from stuns
The second charge does not. It appears once the first charge is full the second charge does not cool down any faster on stunning enemies. This is very blatant as regardless of what the Cooldown is always the same
It appears to be using the remaining cooldown of the first charge, but given it is full, it does not charge any faster