Hi!
I guess the icon size of subgames is handled incorrectly.
I guess the intended icon size is 48px×48px, as this is the size used by the Minimal development test and a lot of other subgames.
However, I figured out that the actual icon size used in the main menu is 47px×46px.
To test it for yourself, I provided an example icon of size 48px×48px with vertical 1-pixel white black and white stripes. Create a dummy subgame and put this file into a folder called “icon.png” and start minetest. You will see the icon is slightly distorted. If you make a screenshot of this and check for the actual size of the icon as presented by Minetest, you will see the actual size of 47px×46px.
I guess it is better to change the presented size in the main menu then to keep the current (weird) size, because many subgames use 48px×48px.