OK EVERYONE, SOLUTION PROPOSAL:
I tried this and got it working.
1.) Start a FaceTime Audio call with a family member or friend (normal call should work too, but I used FaceTime Audio, so I wouldn’t know lol) and talk for maybe 30 seconds or so; it shouldn’t matter.
2.) Click the speaker icon and choose your HomePod mini.
3.) Talk for a few seconds on HomePod mini.
4.) Click the speaker icon on your iPhone and switch the audio back to your iPhone’s speaker.
5.) Try using Handoff to transfer the call back to HomePod mini. Don’t click the speaker this time, try holding your phone on top of HomePod mini.
6.) Be patient, your arm might be there for up to 20 seconds.
7.) The call should transfer. Wait a few seconds, talk, and hold iPhone near HomePod mini again to transfer the call back to iPhone.
8.) Tragically separate with your friend forever my ending the call.
9.) Use a media (audio or video, doesn’t matter) app of your choose and do the Handoff motion back and forth a few times, maybe 3 or so.
10.) Handoff should be working smoothly.
11.) I kindly request that you don’t reply with: “Apple needs to fix this”, “this isn’t ok, it shouldn’t be this hard, or “this isn’t very Apple-esque”, because we all feel the same way and there’s already thousands of other people on the Internet who said literally the same thing. Please just relax and know that Apple will fix it as soon as humanly possible just like with every issue before.
12.) Relax, enjoy some music, and have a chocolate vanilla ice cream cone, because what’s not Apple-esque or human is plain vanilla : )
(Just a joke : ) I know people have preferences)
Hope this helps! If it doesn’t, I’m sorry. Please contact Apple’s best-in-class support team over live chat, iMessage, or phone at 1 (800) MY-APPLE, and an agent from Technical Support will do their absolute best to find a solution that’s right for you.