I think the energy saving is a feature of the phone. On our Apple devices, they show as connected but power saving unless they are actually transmitting or receiving data. I definitely think its more likely a phone issue than an AP issue. A 5G-only SSID will force the phone connect at 5G however I would probably side with the people who asking what the benefit is forcing the phone onto 5GHz unless you are seeing an issue with stuttering video streaming or something like that.
I suspect your correct. I haven't changed it yet. As pointed out there is probably no need and I'm just looking at headline numbers