I found a bug I think. I rebirthed & got no rebirth points.
It doesn't seem to be a bug but rather a feature. First rebirth gives you stronger enemies that give more exp and bigger chance for dropping a piece of equipment. It also raises max lvl you can rebirth on. That's where I am now when it comes to progress but I think it is meant to be like that. I.e. 15 rebirth points unlocks T5 equipment which I believe is strong so these points must be hard to obtain