Hi,
I have the same Problem, sometimes the Phone (FP3) is stuck in Headphone mode.
Now i found an acceptable solution: there are some Apps that give you the ability to toggle between Headphone and Speaker mode. It’s not optimal but way better than rebooting everytime it happens!
For me it is “Lesser AudioSwitch” but there are also other Apps.
Hope it helps some people until Fairphone realeases an Solution.