If you have a reaction that disrupts the trigger, can you cause a creature using Stand to remain Prone?

Question

In a recent bout with lesser deaths, we noticed some particulars about Move Actions that Trigger Reactions. Particularly:

If you use a move action but don’t move out of a square, the trigger instead happens at the end of that action or ability.

However, the lesser death reactions (and some others) can disrupt movement

If the Strike hits, the lesser death disrupts the triggering action.

There are other instances of this, such as Monks' Stand Still feat (on a critical hit). In such a case, is the target rendered still Prone or is the action done and cannot be disrupted?

Answer 1

They Stay Prone

Stand is a simple action: it simply states:

You stand up from prone.

Disrupting actions states:

When an action is disrupted, you still use the actions or reactions you committed and you still expend any costs, but the action’s effects don’t occur.

And the Move Actions that Trigger Reactions rules don't specify anything that would go against the standard disrupting rules. The timing clause has implications for whether the target counts as prone when standing, which you noted in your comment. But the clear and plain reading seems to be that:

1. Disrupting an action prevents the effects of that action.
2. Standing has the effect of removing the prone condition.
3. Disrupting a stand should leave the target prone (even if they are not prone for the triggered attack).

Answer 2

GM Discretion

Disrupting actions describes what happens when actions are disrupted when they have been partially completed at the time:

The GM decides what effects a disruption causes beyond simply negating the effects that would have occurred from the disrupted action. For instance, a Leap disrupted midway wouldn’t transport you back to the start of your jump, and a disrupted item hand off might cause the item to fall to the ground instead of staying in the hand of the creature who was trying to give it away.

So what the effect would be of disrupting the end of a Stand action would fall to the GM to decide.

At the end of Standing a character is likely no longer prone, so the GM may decide disrupting the action doesn't really change things or might instead earn some other penalty in line with the examples above. Or they may decide that the action is completely disrupted regardless so the character never managed to complete their Stand and is still prone.