I found this via google, high in the results and no one had an answer so here is my findings in case they help someone else.
This issue is still in iOS full public release 15.0.2, and breaks hotspot for both windows and older iOS 9 devices.
I have no issue between iPadOS 14 and an iPhone in the family it prompts and joins automatically via the secret apple method.
My guess is lack of, or newer QOS version on the hotspot from limited testing.
The answer on windows which should fix those working remotely:
Find your way into the ‘Network Connections’ right click on your wifi device and select ‘properties’ under which you will see a list of tick boxes. Untick ‘QOS Packet Scheduler’ and click ‘Ok’.
You may have to go into ‘Manage known networks’ and forget the existing connection to your hotspot.
The only settings in that are basic and likely defaults so they can be redone if needed.
password
automatic connection
public or private
random MAC address
metered connection Y/N
Sorry no idea on a fix for older iOS as you can’t control those options. Same for any other device where you can’t set QOS options.
Finger crossed apple fixes this as I know a lot of people who are stuck on iOS 14 until this is resolved as they are all working remotely and can’t take the risk.