Sunstar Chaplain, when you have two tapped creatures at beginning of end step, should allow you to put a +1/+1 counter on a creature you control.
The trigger happens, I select a creature to put the counter on, and play proceeds with no +1/+1 counter on the creature I selected.
This bug has happened multiple times now, in different games on different days.
I've uploaded a log file of a game against Sparky where I reproduced the bug.