There are 2 creatures in the game that can return on their own to hand.
Startled awake
Dust stalker
If these creatures return to hand the next time those are cast they become “immune” to haste. So even with haste won’t be able to attack the urn they are played.
This is particularly annoying with dust stalker since it had innately haste and the bug will make it useless if you have no other colorless creature in play.
For sure it happens when they retrain on their own.
Did not test if it happens if returned by other means also.
To be noted that the specific copy bugs, not all. A new fresh copy will attack with haste the first time.
I don’t know if it has anything to do with this bug in particular, but I’ve seen dust stalker completely disappear from the game instead of going back to the caster’s hand.
Played Startled Awake, which gets buffed by Olivia.
Startled Awake attacks due to Haste from Olivia buff. Startled Awake transforms to Persistent Nightmare.
Turn x+1:
4) Persistent Nightmare attacks & returns to hand.
Turn x+2:
5) Played Startled Awake that had returned to hand in step 4 & it gets buffed by Olivia.
6) Startled Awake does not attack, despite having haste.
Turn x+3:
7) Startled Awake attacks, because it’s no longer affected by summoning sickness. Startled Awake transforms to Persistent Nightmare.
Turn x+3+y:
8) Played a fresh (never returned to hand) Startled Awake which gets buffed by Olivia
9) Startled Awake attacks & transforms to Persistent Nightmare.
Morphis’ bug report is accurate: it only affects those that have returned to hand.