My experience is the same as yours. If I leave 5G turned on my phone insists on staying connected to a 5G network even when that speed is less than two megabits per second in an area where 4G speeds exceed 50 megabits. It takes it the better part of a minute with no signal to fall back to LTE, which means constant connectivity loss when driving. Every six months or so I turn 5G back on to see if coverage has improved but it never has.