I noticed this a couple days back at Home Depot, of all places. Was looking up the locations of stuff I needed to pick up via their website while sitting out in the parking lot and my iPhone kept switching off 5g to hop on some single bar wifi that I couldn’t delete or deselect auto-join.
Eventually just turned off wifi and the problem was “solved” but man this is going to be annoying if it starts happening at the grocery store or something.