I can tell you that from my limited testing with consumer equipment, it seems both my iPad Pro 9.7” and my iPhone XS Max seem to jump around quite a bit, usually to 2.4GHz over the 5GHz band.
I reduced power of the 2.4GHz to a “low” setting and increased the 5GHz band to the highest setting and they still prefer 2.4GHz. Both devices exhibit the same behavior for me.
I have a Ubiquiti AC capable AP in each room of my house, so distance or obstructions aren’t the problems. (I made sure each test was connected to the expected device).
I reduced power of the 2.4GHz to a “low” setting and increased the 5GHz band to the highest setting and they still prefer 2.4GHz. Both devices exhibit the same behavior for me.
I have a Ubiquiti AC capable AP in each room of my house, so distance or obstructions aren’t the problems. (I made sure each test was connected to the expected device).