Thank you all for the full clarification.
@Sailor Vulcan and Exid
The Arena rulebook is perfectly clear. See page 7 and the Codex entry on Initiative:
"You always choose the order in
which events that affect your
creatures and objects occur
during this phase. In the rare
case that a timing issue occurs,
the player with the initiative
decides the order.
Example: You control a Highland
Unicorn with the Regenerate 2 trait
that has a Burn marker on it. Since
the creature has a lot of damage on
it, the Burn effect could destroy
it. You can choose to resolve the
Regenerate trait first or the Burn
condition first."
"Initiative
Players take turns having the initiative, which is noted with an
initiative marker. Whoever has the initiative gets to use their
quickcastaction first during a Quickcast Phase, and gets to
take the first Action Phase during the Action Stage. Also, it is
used to resolve any timing issues."