Please tell us some basic information before asking for help:
Model Name: Zenfone 9
Firmware Version: 32.2050.2050.34
Rooted or not: No
Frequency of Occurrence: Multiple times a day
APP Name & APP Version (If your issue relates to the app): n/a
In addition to information above, please also provide as much details as you can, e.g., using scenario, what troubleshooting you've already done, screenshot, etc.
Occasionally, when in 120Hz mode, the phone will drop down to 60Hz. I've confirmed this using the developer mode setting that lets you see the current refresh rate of the display. This seems to happen after the screen is set to a high brightness after not being touched for a period of time. Setting the brightness to a lower value appears to revert it back to 120Hz. Other users have confirmed this over at XDA (I can't post a link but there's a thread on the Zenfone 9 forum called "Is 120hz refresh rate supposed to drop to 60hz?").
This seems to have started after the most recent OTA update, but I can't be certain.