Johnedwardlow The sarcasm isn't appreciated. The GrapheneOS project, its developers, and the moderation team don't demand community members only say positive things about the OS, but we do try to make sure information here on the forum is accurate, hence the corrections.
As much as I can understand why you think that the correlation here makes it seem like any other explanation improbable, your theory that GrapheneOS somehow broke GPS on Stock doesn't make any sense, to be blunt. Once you flash the stock OS back onto the phone, there's no longer any trace of GrapheneOS on it. So the question is, how can GrapheneOS break GPS functionality when it's not even on the phone? It can't, so I'd suggest accepting that.
The only other remaining explanations would be a hardware issue or something to do with your setup, like the project account suggested, maybe there's something on your end blocking connections to SUPL or PSDS services. Based on your description, this sounds like the most likely explanation. Maybe you're using a VPN or DNS that helps block "trackers" and it's blocking the device from connecting to domains used for A-GNSS. If so, try disabling VPN or DNS temporarily to see if that fixes the problem. Or maybe you've done something to "debloat" the stock OS or changed settings in an attempt to improve the stock OS's privacy. If that's the case, then you can perform a factory reset, then try again without making any modifications to settings or system apps.
Last thought for now: you can try GPSTest to see if the phone/app can see satellites. If it does, then maybe you don't have a hardware problem (but keep in mind hardware problems can be intermittent).