Weird hotspot password on iPhone after it was replaced by Apple
iOS auto-generates a hotspot password by default, mine is a word followed by four digits, just like yours. I suspect that the hotspot password just isn't backed up, so the new phone created a new default one.

It's not an issue really, just replace it with whatever you want. If you want to keep the old one to avoid re-entering it on your devices, but don't remember it, you should be able to find it by going into Keychain Access (Applications > Utilities) on a Mac that connected previously. Double click on the entry corresponding to the hotspot name, and there should be a checkbox for "Show password".