Problem monitor with 1440p and 120HZ

I would look into your monitor and call DELL not Microsoft. I also have a One X and have never had any problems, as soon as select 120hz it automatically drops me down from 4K to 1440p. You must be misunderstanding something about your monitor as there
must be a defect of some sort or your monitor does not support 1440p at 120hz.

For reference, before responding I booted on both my One X's. One is connected to a Samsung 4K QLED and I had no problem switching from 4k 60 to 1440p 120. My other X is connected via an LG 4K monitor and again when selecting 1440p OR 120hz it automatically
selects 1440p or 120hz without any issues.

Until you don't proof me this specific monitor (Xiaomi Mi Curved 34) enables 1440p@120hz with Xbox Series X, your comment is meaningless.

Yeah, there are some monitors with 1440p@120hz support for Xbox series s/x, that simply doesn't mean that ANY 1440p+ monitor with 120hz+ (especially 144hz+) can do it. Period.

I've tried everything, that (1440p@120hz) simply doesn't work with that monitor. I've already listed every step I've tried, I'm not gonna write it again.

Please read previous comments before posting the same dried comments...

No, HDMI 2.0 is enough for 1440p at 120Hz. HDMI 2.1 is only needed for 4K at 120Hz or higher. Although, with certain monitors, 1440p and 120hz can be tricky and not automatic.

What is your monitor model? There could be specific settings (i.e. 'freesync' and 'allow 4k') that could be interfering.

Also you'll need a HDMI 2.1 compliant Monitor to get 1440p with no problems.

HMDI 2.1 is only needed to get 4k 120Hz+....1440p 60Hz will run fine on HDMI 1.4 as that particular monitor has been designed to do

Buying a HDMI 2.1 monitor for 1440p would be a waste of money

The Xbox One X supports 120hz only at 1440p and 1080p resolutions. It does not support 120hz at 4k. So unless your TV or monitor supports 1440p resolution (many don't), you will only get 1080p when selecting 120hz.