Apparently "basic" clothes have stats that can only be replaced with other clothes, so no new stats are added.
And that's why we can buy the basic clothes in the store, to change those stats if we want. (Except naomi's skirt that is not in the store).
That's my conclusion.
Yes, it's a bug, my base character has more hp for not wearing clothes, and each garment without effects (bottom and legs) subtracts hp.
In the images you can see how I get more hp if I take off his clothes.
I have 150 on average but sometimes it goes up to 250-300 for a few moments, when items appear near me (hp and reload) I freeze for a moment. [Only for items that the game places automatically]