ISSUE SUMMARY:
When Betor, Ancestor's Voice returns Grist, Voracious Larva from the graveyard to the battlefield, Grist's enter-the-battlefield triggered ability does not trigger in MTGA. This appears to be a bug, as the interaction should work according to the Comprehensive Rules.

EXPECTED BEHAVIOR:
Grist, Voracious Larva has the following triggered ability:
"Whenever Grist or another creature you control enters, if it entered from your graveyard or you cast it from your graveyard, you may pay {G}. If you do, exile Grist, then return it to the battlefield transformed under its owner's control."

When Betor, Ancestor's Voice's end-step ability returns Grist from the graveyard to the battlefield, Grist IS entering from the graveyard, which satisfies the trigger condition. Therefore, the ability should trigger, allowing the player to pay {G} to transform Grist into Grist, the Plague Swarm.

ACTUAL BEHAVIOR:
In MTGA, when Betor returns Grist from the graveyard to the battlefield, Grist's ETB ability does NOT trigger at all. The creature simply enters as a 1/2 creature with deathtouch and remains on the battlefield without offering the option to pay {G} to transform.

STEPS TO REPRODUCE:
1. Have Grist, Voracious Larva in your graveyard.
2. Control Betor, Ancestor's Voice and meet its end-step conditions (have gained and lost life during the turn).
3. During your end step, use Betor's ability to target Grist in your graveyard and return it to the battlefield.
4. Observe that Grist enters the battlefield but its ETB trigger does not go on the stack.

CARDS INVOLVED:
- Betor, Ancestor's Voice (TDC #1)
- Grist, Voracious Larva // Grist, the Plague Swarm (MH3 #251)

RULES BASIS:
According to CR 603.10a, a zone-change trigger triggers when an object moves from one zone to another. Grist's ability is an enters-the-battlefield trigger with an intervening if condition ("if it entered from your graveyard"). Since Betor's ability moves Grist directly from the graveyard to the battlefield, the condition is met and the trigger should fire.

Additionally, per the official rulings on Grist, Voracious Larva: "If Grist, Voracious Larva leaves the battlefield while its triggered ability is on the stack, you can't exile it from the zone it's put into, so you won't return it to the battlefield transformed under its owner's control." This ruling confirms that the card IS intended to have this trigger work when entering from the graveyard.