Same problem in Bilbao, Spain, with + 11º C. Send it to repair to Apple 6 times, received 4 replacements, from different Apple storehouses of Europe. All the iPhones that I’ve received had the same problem. When you receive the new replacement and you notify again the issue to Apple with the new iPhone that you have received, they don’t answer your mails. So you have to create a new case and tell all from the begging to the new assistant.
Put a complaint on the Store, official distributor of Apple. They only give u the option to send it again to get a new replacement, but it will have the same issue. Video of the flash not working https://m.youtube.com/watch?v=ZQu_Xh0vlDQ
Please sign the next request that I have notified in changes.org to have a solution to this issue. https://www.change.org/p/apple-iphone-8-iphone-x-flash-cámara-no-funciona-cuando -temperatura-ambiente-inferior-a-11-c Share it.
Thanks
tested in iPhone 8 plus 64 Gb & iPhone 8 Plus 256GB
also happens in iPhone X
Totally disappointing