I got a merge conflict while merging code from my dev to my nonprod

I got a merge conflict while merging code from my dev to my nonprod.

My gitlab project branches overview:

On my project I have master branch,non prod branch, dev branch( Here all of these 3 branches are identical).Created feature branches every-time from dev branch.Did commits on feature branch and merged back to dev branch, created new feature branch from dev branch every-time.

Background about my work:

I created featurebranch_1 from my dev branch.I did a couple of commits on featurebranch_1 and merged back to my dev branch.Again I created featurebranch_2 from my dev branch and did a couple of commits on featuerebranh_2 and merged back to my dev branch. This process I repeated for creation of featurebranch_3,featurebranch_4,featurebranch_5 and every time merging back to my dev branch from feature branches.Code merged successfully. But, merging code from my dev to my non prod branch I got a code merge conflict. Here my dev branch and my nonprod branch both are identical.

Kindly provide your solutions to resolve this kind of issue and your thoughts about it, why is it happening ?