MASSIVE FPS drop (half) when playing on 4K TV

CPU: Intel Core i7 4790K 4.00 GHz, no overclock
GPU: GTX 1808 Ti, 11 GB VRAM, No Overclock
RAM: 32 GB DDR3 1333 MHz
OS: Windows 10 Enterprise x64

Description of the problem:

A bit long post because I have tried everything I could before posting.

I have been playing NieR: Automata on my computer for a while now. As I have found out from an earlier discussion, the reason my 144 Hz refresh rate monitor is always at 60 Hz is the game has a bad "PC port". OK, fine. No problem. I am OK with 60 FPS.

However, I have a Samsung Q6FN 4K TV with max refresh rate 120 Hz. I like playing on the big screen since I am not really into competitiive high FPS games but just RPGs and care about the visuals more. I only played on it earlier and it was fine I remember.

However, today, when I tried playing it on the TV again after months, I found it was VERY blurry and lagging and just not even playable. Bizarre. I did notice the graphics had the option of 4K resolution in the game menu so I set it to that. I thought that's the reason because my 1080 Ti can't handle 4K @ 60 FPS. But nope. It's stuck at a perpetual 30/31 FPS when I use nVidia in-game overlay to check.

I tried everything from lowering resolution to 1440p and even FHD and also graphics presets from high to medium to low. NOTHING worked. Except the window. I changed from fullscreen to Windowed mode and IMMEDIATELY it shot up to 60 for both 1440p and FHD and even 45 FPS for 4K. I switch back to fullscreen and it immediately locks back at 30 FPS. I have tried everything in the main display settings of Windows, nVidia control panel, whatnot.

I then checked on my computer again to make sure it's not a problem with the game itself and it's still running on my computer at 60 FPS. So it's definitely a TV issue. I tried even an 8K HDMI 2.1 cable but no use so the DP to HDMI cable is not the bottleneck. I tried game settings on and off on the TV. No change. I checked many forums on the internet but 90% of them are about fullscreen INCREASING FPS not decreasing it. I have literally exhausted everything and just want some help. It really shouldn't be too hard to play on 60 FPS at 1440p. I do it on the computer all the time. Why can't it just replicate it on the TV like it does on the computer monitor?

Thanks for all your help!