Some basic information before asking for help:
Model Name: ROG Phone 5
Firmware Version: RKQ1.201022.002.WW_Phone-18.0840.2106.86
Rooted or not: Not rooted
Frequency of Occurrence: On every use
I think ASUS implemented One-handed mode with this update.
But the problem or bug maybe is that it resizes itself to full-height after a few seconds on it's own.
I tried using this mode on a variety of apps and got the same experience.
