Re: Pen leaking into barrel
That’s a tough one to diagnose. A very straightforward cartridge pen that leaks into the cap or barrel… I’d be using good illumination and magnification to look for any cracks. Somewhere there is an air leak that is allowing ink to flow on it’s own.
Re: Pen leaking into barrel
I've found images of the Ensso XS Minimalist Pocket pen and from what I've seen they are all metal. It's a cartridge only pen, so no problems with a converter.
From the OP I got the idea that ink was leaking from the back end into the barrel rather than the front end of the section leaking into the cap.
I would not try to force the housing out as it's likely that would completely break the pen and invalidate any warranty.
The next thing I would check is the cartridge piercer/nipple. i.e. is it still there in it's proper place? It may have broken off/cracked/split inside the housing or inside a cartridge. Did the brand new cartridge you inserted get pierced and feel like it fit tightly onto the nipple as normal?
As a starter I would completely clean out the pen and whizz the section a few times in the USC.
If a brand new, tight fitting, cartridge gets pierced but then leaks from the back end of the section behind itself and into the barrel while the pen still writes then this problem sounds like a question that you may need to ask the retailer or manufacturer.