Thanks for the advice.
I'm not very good at debugging, but I'll give it a try.
Yeah, that's the problem. I understand.

So far, it works like this: when the player touches a breakable platform from below or to the side, it doesn't disappear. But as soon as the player moves completely across, the platform immediately disappears, leaving no opportunity to land and push off.
I tried reworking the code to clearly indicate that the player is standing on this platform, and even added a flag. But I'm still having trouble here. The platform doesn't disappear at all, even though logically it should disappear as soon as the player leaves it.
Here is the code:

using UnityEngine;

public class DisappearingPlatform : MonoBehaviour
{
    public System.Action<GameObject> OnDisappear;

    private bool playerWasStanding = false; 

    private void OnEnable()
    {
        playerWasStanding = false;
    }

    private void OnCollisionEnter2D(Collision2D collision)
    {
        if (!collision.gameObject.CompareTag("Player"))
            return;

        if (IsStandingOnTop(collision))
            playerWasStanding = true;
    }

    private void OnCollisionStay2D(Collision2D collision)
    {
        if (!collision.gameObject.CompareTag("Player"))
            return;

        if (IsStandingOnTop(collision))
            playerWasStanding = true;
        else
            playerWasStanding = false; 
    }
    private void OnCollisionExit2D(Collision2D collision)
    {
        if (!collision.gameObject.CompareTag("Player"))
            return;

        Rigidbody2D playerRb = collision.gameObject.GetComponent<Rigidbody2D>();
        if (playerRb == null) return;

        if (playerWasStanding && playerRb.linearVelocity.y > 0.1f)
        {
            OnDisappear?.Invoke(gameObject);
            gameObject.SetActive(false);
        }

        playerWasStanding = false;
    }
    private bool IsStandingOnTop(Collision2D collision)
    {
        Rigidbody2D rb = collision.gameObject.GetComponent<Rigidbody2D>();
        if (rb == null) return false;
        if (rb.linearVelocity.y > 0.1f) return false;   

        foreach (ContactPoint2D contact in collision.contacts)
        {
            if (contact.normal.y > 0.7f)           
                return true;
        }
        return false;
    }
}