Does anyone know what ‘bifrost.bundle’ is on iPhones?

The solution: reset your location and privacy settings and it is gone (thanks, Reddit):

If you would like to reset all of your location settings to the factory default, go to Settings > General > Transfer or Reset [device], tap Reset, then tap Reset Location & Privacy. When your location and privacy settings are reset, apps will stop using your Location Services information until you grant them permission.

About privacy and Location Services in iOS, iPadOS, and watchOS - Apple Support

I also had bifrost.bundle sharing my location and no clue where it came from. Maybe Proton VPN, Metamask or Monopoly Go, since these are the most exotic apps I had. I only found about it when checking location sharing after I had set up an iPhone with everything transferred directly from another iPhone.

When you did the factory reset, did you set it up as a new device with only your Apple ID and not by restoring from a backup or transferring from another device?

Got any add-on cryptocurrency or wallet or DeFi or VPN apps loaded? If so, remove all that, restart, and check again.

We are all just users like you.

One issue is Bitfrost could be many different things.

The burning rainbow bridge from Asgard to Midgard, software in Autodesk for videos, Bitfrost AI used to generate images from map data, a crypto coin network, etc.

Do you play Assassins Creed Valhala on your iPhone? From what I found in a google search, this appears to have some connection to this game.

Well, start removing apps one at a time, until it goes away. Then you’ll know what app it was associated with.