Thanks for your swift reply.
The callback function is the function run after the accounts are run in parallel. Previously this function was run as is the manager account where the script is being run was the selected account; now it is run as if the final account to be run was the selected account.
It seems like a pretty simple bug - sometimes which account is selected when the callback is run would not make any difference and I imagine that is why it evaded your testing.
I've attached a script which shows the behaviour and included a screenshot of the script and the logs below.
Cheers!
Tom