You may notice that when you highlight an item, a purple border texture appears. the same works in Java Edition. However, in Bedrock Edition the purple border is not transparent, making it impossible to see anything over the purple border texture.

Steps to replicate

1. Pick any item/block.
2. Move the mouse over said item, and notice the purple border texture.

Expected

The purple border is transparent, like in Java Edition.

Actual

It is fully opaque.