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.

I bought my Pro5 in first week of dec, 2016. This was an Android version, originally. I installed Touch on one-day device. I have used Touch until apr of 2017... after it I reinstall Android...
pro: I love GUI of Touch, no ads/spy/etc, convergence was amazing, base apps are well designed and easy handling of base apps.

contra #1, overall/base: bugs and GUI/phone random reboots, no notification services
contra #2, ~400EUR phone point of view: missing apps (you know what I mean...from offline navigation to social media)
contra #3, "come on guys, it is 2017!!!" point of view: no online banking, online pay (parking/road toll/tickets/food/etc), NFC pay

So, if you have base reqs for a phone then you can use Touch, as I used it. You will love it. But before install Touch look around what apps do you need and are there required apps for Touch.

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

@darkeye would you summary your experience? Should I switch back to UT?

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

@Lars which CM do you use? Would you send me a link? I am lost, because I have a ~400EUR first class hardware and no useable software:
#1 RR AOSP Android: very-very unstable RIL and fingerprint unlock, autoplay music when earphone unplugged
#2 Flyme: autoplay music when earphone unplugged (loud music!), spyware
#3 Ubuntu Touch: khmm, you know...

@Stefano is there fingerprint support in rev2 of pro5？

@NeoTheThird Thanks for info! Based on my experience, the fingerprint unlock of PRO5 (and its stability with UT) is the almost representative feature (the wooow factor) next to Scope design. Android AOSP ROMs have very, very unstable fingerprint unlock - I think, they can not use original, Flyme driver/solution (?).
I am looking forward the Ubports news, I need a little push to switch back to UT...

@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...?

@darkeye is fingerprint security working on Ubport image?