My iPhone keeps merging contacts when two of them are of the same company or have something else that is identical.
So, let us say I wish to call a contact (that unknowingly to me has been merged), I keep scrolling through my list of contacts until I finally decide to obtain their number via other means. Much later, I find that this very contact had been merged with the main company contact or one of their colleagues.
It has happened countless of times since the iPhone 5. Unmerging and re-adding contacts has become part of my routine.
My iPhone has one Exchange account configured that synchronises the contacts. I use Outlook 2016 on PC.
How to stop my iPhone from merging contacts?