It’s definitely the bottom module that’s the problem. My wife has an FP3 as well, and when I swap out the bottom module in my phone for the bottom module from her phone, my phone charges perfectly.
I don’t think I’ll be covered by the warranty because my bottom module stopped working after I accidentally dropped my FP3 into some mud while gardening and mud got into the charge port. I’m just looking for a bottom module so I can make the repair myself.
[Edit to add: I have tried carefully cleaning out the charge port and using contact cleaner, and it’s still not working, so I’m assuming that my problems are caused by damage to the contacts rather than residual dirt in the charge port]