I am now on my third Fairphone (FP4) across over ten years and for the first time ever I have a cracked screen. I actually got two cracks in less than a year of having it, and another has just appeared, even though at no point was there a noticable bump or dropping of the device to make the cracks. I am wondering if there has been a drop in quality of the screen in this version, hence the easy cracking, and if anyone else has noticed this? I was about to order a new screen (I’ve contacted FP about warranty but it’s been weeks and I’ve not heard back), but am not sure there is much point if it’s just going to be as susceptible to cracking (I have a screen protector which is working to hold it together at the moment). Can anyone support my theory?
I can only offer a data point for the opposite experience.
I’ve used my FP4 for 4 years without a case or screen protector, it’s hit hard floors a few times, the only part I had to replace was the plastic back panel at some point (but I also took that off at least a hundred times to get at the sdcard).
So no, at least for my device, there doesn’t appear to be a quality issue that makes the screen more prone to developing cracks.
I havent read reports that would support this and mine is fine, although I dont use it as main phone any more, its now 3.5 years old and used without case.
In my case I have already replaced the screen twice, but the cause was the stupidity of the owner, not the quality of the screen in every case 
No issues for me, and I use it without any kind of external protection, too.
My FP4 is almost 4 years old, has no protection whatsoever and I dropped it from waist height on the street once or twice and quite regularly from chair height to a tile/vinyl/thin rug floor. No cracks so far.
No cracks here either and I am just two days apart from my four year badge.
But if there is already a crack, the stability of the screen is gone and the next cracks happen much faster. So, no wonder you also got crack number two and three.