Square corners when tiling using super + T with 0 padding

Hello,

When I tile using the screen edges, the corners get squared not to show slivers of the background:
image

This is however not the case when using super + T with 0 padding:
image

How can I fix it, and is it a bug ?

P.S: What’s the name of the super + T tiling feature ? I couldn’t find any documentation