3 official rulings from Bandai, last ruled Mar 28, 2024. Straight answers to how this card actually resolves.

This card's <When Digivolving> effect reads "You may place 1 card with [Royal Knight] in its traits and a play cost of 13 or less from your hand or trash under this Digimon as its bottom digivolution card. Activate 1 of that card's <When Digivolving> effects as an effect of this Digimon." At what timing does that <When Digivolving> effect activate?
It activates immediately after you place the card with [Royal Knight] in its traits and a play cost of 13 or less as the bottom digivolution card. The placed card's <When Digivolving> effect activates as part of this card's effect, and after its processing has completed, you can declare an attack for this card's <Blitz>.
Ruled Mar 28, 2024
This card's <When Digivolving> effect placed a [BT10-016]/[Jesmon (X Antibody)] as a digivolution card, then that card's <When Digivolving> effect played a [BT6-082]/[Sistermon Blanc], then I activated <Blitz> in this card's <When Digivolving> effect to attack. When does [BT6-082]/[Sistermon Blanc]'s <On Play> effect activate?
[BT6-082]/[Sistermon Blanc] is treated as a card played by this card's <When Digivolving> effect, so it is triggered at the same time as the <When Attacking> effect that was triggered by the attack declaration for <Blitz>. The player may choose the activation order for this Digimon's <When Attacking> effect and [BT6-082]/[Sistermon Blanc]'s <On Play> effect.
Ruled Mar 28, 2024
This card's <When Digivolving> effect placed a [BT10-016]/[Jesmon (X Antibody)] as a digivolution card, then that card's <When Digivolving> effect played a [BT6-082]/[Sistermon Blanc], then I activated <Blitz> in this card's <When Digivolving> effect to attack, and when the <When Attacking> effect deleted one of my opponent's Digimon, my [BT6-082]/[Sistermon Blanc] was deleted by the <On Deletion> effect of the opponent's Digimon. [BT6-082]/[Sistermon Blanc]'s <On Play> effect hasn't activated yet, so does it activate?
No, it doesn't. [BT6-082]/[Sistermon Blanc]'s <On Play> effect is triggered, but it doesn't activate because the card is deleted before it activates.
Ruled Mar 28, 2024
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