[2019] Machina Mitts Gloves not working as intended

After a lot of testing i concluded that this item must transfer life leech instances from mines to you. Means that if you detonate a mine that hit enemies you only get life leech if you already have UNdetonated mines on the ground at the same time

That is definitely not at all how it works. The only leech instances involved in this mechanic are applied directly to you, not to the mines, and leech instances cannot be moved from one thing to another.

If for example you simple throw a mine and detonate it you ll never get life leech.

This is incorrect. A single mine can and does correctly apply a leech instance to the player for each thing it hits. I have tested this in-game and it is working correctly. The amount of the leech will be low from an individual mine, but it does occur.