For everybody:
I also run into some issues with my free available carrying capacity (despite keeping only the minimum equipment, what I really use). It constantly decreased as I progressed through the game, and I thought it was a bug.
However I discovered that it was caused by the grenades. The issue is that their weight (standard 0.3) what is displayed is the per unit weight, however you stock multiple items, thus 100 pieces will weight 30, not 0.3.
It is logical, but not necessarily obvious. It tricked me certainly. I just decrafted a lot of grenades and regained a lot of free capacity. Some of them cannot be decrafted (the game disables the decrafting of that item if you do it too much, because it believes you want to exploit it by crafting and decrafting a mass of items) - I will need to throw away them.....
I hope it helps others too.