Heliod's triggered ability prevents creatures from dying to state-based actions while on the stack
Heliod's triggered ability to place +1/+1 counters on creatures overrides creatures dying to state-based actions while it's on the stack. In the paper game, if Heliod's controller attacks with a creature with lifelink and that creature is blocked and dealt lethal damage during combat, Heliod's ability will go on the stack, then state-based actions will be checked and the creature will die, fizzling the ability. on MTG Arena, counters can be placed on the creature to save it from lethal damage, despite combat damage resolving and state-based actions being checked once all the Heliod triggers are placed on the stack.
You can cast spells while Heliod's ability is on the stack, which means state-based actions MUST be checked before it resolves, so it's clearly not a case of the ability incorrectly resolving as a replacement effect or something of that nature - Heliod's ability simply makes your creatures completely immortal until it resolves, which is very, very obviously NOT intended behavior.
Attached is my log file and a screenshot of the game in which I was in this exact situation. You can see that I have priority which means state-based actions have been checked while the opponent's creatures are dealt lethal damage, yet none of them go to the graveyard. This happened multiple times prior to this screenshot as well.
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[Deleted User] commented
You do realize that all your opponent's creatures have indestructible and so can't be killed by damage in the screenshot you posted, right?