On Bedrock Edition, you can only attach torches, levers, and buttons on one side of the corner stairs. This is possible on Java Edition, which can place on two sides at the back of the corner stairs. This bug report is related to MCPE-33359, which has already been fixed but only on the back of the stairs that aren't the corner stairs.

The following blocks are affected:

• Torch
• Soul Torch
• Redstone Torch
• Buttons (all types: wood, nether wood, stone, and blackstone)
• Levers

Step to Reproduce
Try place the mentioned blocks behind two back sides of corner stairs

Expected Results
You can place it on both back sides of the corner stairs.

Observed Results
You can only place it on one back side of it.