Trying to figure out some weirdness with my iron sights

So in the last few months, I assembled a G36 parts kit with genuine German parts on an S5 clone receiver. The build has run very well with one small exception. The irons I have run on it are very far off alignment.

Originally, with the plastic G36C top rail with fixed sights, the point of impact was so far low and to the right that it was almost completely off paper at 25 yards. Upon mounting my Vortex UH1 the dot was also very far off, but I was able to zero the sight to correct for this. The rifle seems to group consistently, so I’m confident the barrel is installed properly. I also had a few friends at the range test it out to rule out my own error, and they consistently had the same problem.

At this point I chalked it up to possibly just being a crappy set of irons that I’d likely never use, and the rail did hold zero with the optic, so it wasn’t a huge deal in my mind.

Fast forward to today, I installed a new IDZ rail with the flip up sights and had the same issue when I went to dial in my sights. Way low and to the right, albeit a little less severe, still enough that I’d qualify it as a problem. Cranking the rear sight all the way to the left seems to have mostly fixed the matter of left/right point of impact, at least at the 25 yards indoor lane I was troubleshooting at.

Just like before, the vortex holographic sight is holding zero after adjusting it to be on target.

I guess what’s confusing me is mostly that I’m unsure what the issue may actually be. This G36 clone build is the first rifle I’ve actually fully assembled myself, so I’m thinking it may just be something I’m inexperienced with that’s causing the sights to be so far off, but I would appreciate any insight as to what the issue may be and how to solve it.