When Minecraft Bedrock received the new textures update, pistons were not updated proprely because of the different way the arm is render on Bedrock.

The piston texture block item itself is updated but not the block when placed.
Here are how it looks in game currently with the current texture with also how it should look like with a proper fix for normal and sticky version.

"pistonArm" and "pistonArmSticky" are the correct and updated textures.