Hi, I had same issue on my Pro5 in dec. 2017. I had switch back to Android to doublecheck but no change. After it I tried to "cold down" method: I placed the phone to fridge (no freezer) for 2-3 minutes and then the "on headphone" sign had gone. it works fine until next reboot or headphone usage.....
My phone was under warranty, so I could send back to seller to repair. They did not/could not give back the pro5, I could choose other brand new phone because there was not pro5 in the shop anymore.
I think, this is a hardware issue and need to repair somehow. I agree with @advocatux , this fridge/cold down method is not healty for phone.

@darkeye , thanks for info! Hmmm...., this fact means for me that Ubports image is not same as original, stable release...
What is the reason? Fingerprint driver is not open source? Had been mirrored from not stable image - I mean rc-proposed or something...?

@Stefano would you summary your experience about UBports image on pro5? Do you use as daily driver?

@darkeye , @Stefano , @Lars Hi, thanks for info! I am looking for more news, may I switch back to UT.