Steps to reproduce:

Create a setup like in fig1.png

Run /tick freeze

Break the redstone block followed by the piston head (the order is important)

Place a piston against the oak planks so it faces downwards

Compare your setup to fig2.png

Run /tick unfreeze

Compare your setup to fig3.png

Observed behavior:

The block below the piston breaks but doesn't drop anything

Expected behavior:

The block below the piston breaks and drops a block according to the rules of /setblock <coordinates> air destroy

Suggested fix:

On a piston retraction if the block in front of the base is a minecraft:pison_head do nothing and continue to retraction logic,

Otherwise, break the block in front of the base rather than setting it to air

Java-like pseudocode:

public class PistonBase {
    public void onRetract() {
        // If the block is a piston head, do nothing
        if(this.getBlockInFront() instanceof PistonHead)
            this.getBlockInFront().getLocation().getBlock(AIR);
        // If the block is not a piston head, break it
        else
            this.getBlockInFront().getLocation().destroy();
        // The rest of the logic in this function
    }
}