[2022] possible bug: MTGA does not allow "overpaying" casualty for Ob Nixilis, the Adversary

TL;DR: Ob Nixilis, the Adversary seems wrongly coded on MTGA. The client wrongly prevents the player from "overpaying" the casualty cost and getting less loyalty on the copy than the sacrificed creatures power.

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The comprehensive rules entry on casualty state (702.153a, emphasis mine):

Casualty N means “As an additional cost to cast this spell, you may sacrifice a creature with power N or greater,”

[[Ob Nixilis, the Adversary]] has the ability:

Casualty X. The copy isn't legendary and has starting loyalty X.

It seems to me, that the following should be a legal play:

1. Cast Ob Nixilis, declaring X to be 0.
2. Sacrifice a creature of power 1 to pay for the casualty cost (since casualty 0 means I can sac a creature with power 0 or more).

In other words, it should be possible to have the token copy of Ob Nixilis enter the battlefield with _less_ loyalty than the sacrificed creatures power. However, I was not able to accomplish this on MTGA. Even in full control the client does not allow me to specify X but automatically sets X to be the power of the sacrificed creature.

This behaviour is consistent with the MTGA reminder text for Ob Nixilis which states:

Casualty X: As you cast this spell, you may sacrifice a creature with power X. When you do, copy this spell.

Note the absence of "or greater", which is present in the comprehensive rules.

Of course I am aware of the fact that this bug is going to be irrelevant in 99.99% of cases, since you almost always are going to want the loyalty of the copy to be as big as possible. However, there are niche situations where it might matter, such as when I want to create a 0 loyalty Ob Nixilis to get a death trigger for [[Cruel Celebrant]] but only have creatures with 1 power to sac.

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EDIT: Some people are referring to the reminder text of Ob Nixilis to argue that Casualty somehow works differently in this case and that X is always determined to be the power of the sacrificed creature. I would like to remind everyone (no pun intended), that reminder text is not rules text and is not always 100% accurate. Sometimes the reminder text might be slightly wrong for simplicity, especially in a situation like this when the difference does not matter in 99.99% of games. When in doubt, the only thing that matters are the comprehensive rules. From the comprehensive rules (italics mine):

Reminder TextParenthetical text in italics in the text box of a card that summarizes a rule that applies to that card, but is not actually rules text and has no effect on play. See rule 207.2.

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EDIT 2: Here is how to cast Ob Nixilis according to the rules. For a more detailed analysis, see the comment linked below.

Step 1. Move the spell onto the Stack.

Step 2. Declare that the spell is being cast with Casualty. Declare the value for X.

Step 3. Pay the costs. This includes the mana cost and the additional Casualty cost which involves sacrificing a creature with power N=X or greater, where X is the value chosen previously.

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EDIT 3: For everybody interested in a full analysis of the rules regarding this tricky situation, please see this comment by u/Douglasjm

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