This seems common. We have two iPhone 12 Pro Max's both have the same Hotspot problem. 50+Mbps on phones, 1.0 - on hotspots. Tried laptop, and two iPads. Same. Tried restricting Hotspot to LTE, Compatibility On/Off, Tried different locations. All same result.
Called T-Mobile and got transferred to a good technician who discussed the local cell towers capability, one is 5G/LTE and other is just LTE. LTE is being repaired so he suggested that was the problem and to wait till it was fixed. However that does not explain why it was the same at the T-Mobile store in the local city. Different Cell towers there.
I called Apple and their response, "Not our problem, it's a cellular problem". Same response when I argued, politely, that if the phone was seeing 50Mbps but only outputting 0.5Mbps then it could be an Apple problem.
Other fixes I tried. Resetting the phone, (volume down, volume up, hold power button down until Apple logo appears), at instruction of T-Mobile. Restarting both phones and iPads.
So I am unsure who else to try. I will recall T-Mobile when I get an hour free! Meanwhile I hope someone else finds a solution and posts it as I will if I find one.
On the good side the phones 5G performance is better than I expected, My iPhone 10 on LTE was 22Mbps, Wifes iPhone XS MAx was 29, iPhone 12 Pro on LTE is 33 and on 5G is 53Mbps. All the average of eight download test on Speedtest.Net. I have seen a max of 166Mbps on the 5G.
Today I am getting the T-Mobile Home Internet Modem delivered. That will keep me distracted from this problem. Will be interesting to see what speed that gets.