I assume this is either a driver or Win 10 issue, but since I am not sure, I am here.

I have two computers and this only seems to be happening on one of them and I am not sure why.

Issue: Gsync doesn't work on DX12 mode of BF5 or Rise of the Tomb Raider on one of my PC's. In BF5, it works fine in DX11 mode (which is hurting my FPS by about 30%, so I don't want to run DX11) and the Hz the monitor shows is exactly what I set in the advanced graphics menu. For instance, if I set 2560x1440 165 FPS, my monitor shows 165 hz, if I set 2560x1440 at 144, the monitor shows 144 Hz. And I am getting in the 90-110's FPS for reference. In tomb raider, it seems to just lock at 165 as there are no advanced settings to chose from. I have vsync off in the games, and both work as expected in DX11 mode.

I have fast sync turned on in control panel, everything else is default. I have tried DDU twice, once with newest drivers (417.22), one with last version (417.01). I am not sure if this issue has been there previously as I usually leave the monitors Hz overlay off.

PC showing issue is a i7 3770k @ 4.6, GTX 1070 FTW, Win 10 1809 with a Predator xb271hu.
PC seemingly without the issue is a i7 4700k @ 4.6, GTX 1080 FTW, Win 10 1809 with the ultrawide Predator, model number I am blanking on, 3440x1440.

Any ideas?

Since you have the game(s) working on one PC with DX 12 then it is not an issue with the game itself. I would believe it might an issue with Windows. Open a CMD prompt with admin rights. Run sfc /scannow

Just to clarify but every render pipeline behaves differently and is it's own thing. DX9, DX10 or DX11 are not the same render pipeline as DX12. You'll find things like SLI and G-Sync need to be implemented at a driver level to interface each render pipeline version. Never mind that Microsoft loves to randomly change the desktop render pipeline randomly every 6 months in thier Windows 10 updates. I'm surprised anything works right in Windows 10 environment with Microsoft development methods.

I personally think its just how unoptimized the coding is. When DXR was released in BF5 all the major reviewers were running tests in 1080p due to "low frame rates" and were getting 70-80 fps it seemed. I decided to just see how bad it was and set it to 1440p on my computer. I actually saw the same range. This puzzled me as it wasnt a decrease, I was expecting one since I was set to a larger resolution. I was not over clocked on the GPU or CPU at the time, I was stock speeds. I dont have much to back it up past my own testing. I just think the game was rushed, the Nvidia launch was rushed, and everything is just poorly optimized. I know you arent running a 20 series card but it wouldnt surpise me if these issues are somewhat related just to poor optimization on basically everything DX12.\
 
I work in the software product validation world, and let me tell you. Legacy devices might have been working fine, but as soon as you split attention by introducuing a new product line, the number of issues that get through increases. If you dont increase staff, then the original staff basically have to do their original work and then some. Even if you do add new staff, unless they were in from the ground level, theyre almost a hinderance at release because chances are they dont know the ins and outs of your proprietary software. Thus they need to be trained, which splits attention even more. Anyways, this leads to everything being rushed, leading to more bugs that getting through....and it only compounds. Unfortunately the best thing to do is to slow down a bit and sort things through....which makes everyone more upset because issues are not getting fixed as fast as theyd like, both customers and "the man uptairs". Which obviously only increases the pressure on the team. So Id say, even if you dont have the new card, its quite possible issues have worked their way into legacy devices in the recent updates. Paired with the rush implementation of items on Nvidias end and the game devs.
 
Id say to not only users who have the 20 series cards, but all Nvidia card owners. Report your issues, the dev team cant fix what they dont know about, plus it helps assign priority for fixes. Also to expect some issues to arise and be patient for the fixes. Not saying dont investigate items when they go wrong and write them off immediately as Nvidia problem either though. Just be patient for the next few months.

Problem still exists for me :/. From what I have read, sounds like nviida is just ignoring gsync issues as others have had this issue for a while. I guess time will tell ;(