I found out that light can pass through piston blocks on Minecraft: Bedrock Edition while I was building my Monster Spawner farm today. When I searched this issue up on the web, I found out that this was also the case for Minecraft: Java Edition, until it was eventually fixed. Hopefully, the same fix gets applied to Minecraft: Bedrock Edition in the future, since it makes more sense for pistons to be treated as opaque blocks rather than transparent blocks. I'll link a Reddit post that I found really helpful below:

https://www.reddit.com/r/Minecraft/comments/9xfgdd/pistons_do_no_longer_let_light_through_since/

The screenshot that I uploaded with the name "Screenshot 2023-08-01 at 8.44.03 PM.png" shows that the piston appears to be self-emitting light, when in fact, it is just the light from the lantern in the room behind it.
The screenshot that I uploaded with the name "Screenshot 2023-08-01 at 8.43.24 PM.png" shows that the light from the lantern is indeed passing through the piston and leaking to the chamber containing the Monster Spawner (thus preventing any monsters from spawning).

Edit: I just tested out this issue in my Java Edition copy of the game, and here's what I found. If the light source is at the back of the piston, light is greatly diminished (I say "greatly diminished", because a very teeny amount of light still passes through) for both the deactivated and activated states. On the other hand, if the light source is at the side of the piston, light is greatly diminished for the deactivated state, but passes through for the activated state. Personally, I think that this logic makes more sense, and would love to see that in Minecraft: Bedrock Edition.