I've had this issue for a couple of months now. I shoot pictures on my Canon 6D MK II and use the .CR2 (RAW) format to edit them in Lightroom Mobile. I started using this method in January 2021 and it was all going fine until June 2022, when my RAW-files suddenly began to appear much darker than how they were displayed on the camera monitor, my pc, and my mobile phone's gallery. I have no selected presets for the import and I don't know what my camera could have to do with all that as I have changed no settings for at least a year. Meanwhile, JPG-files remain unchanged and the support didn't really help me.
I added an example below. Thanks in advance and have a nice day, Moritz.
Have you looked at the phone Display & Brightness settings? They might have changed without you knowing with an iOS update. Depending on which phone model you are using and what version iOS you have you should check the settings for 'Display and Brightness'. I have a 13 and do not use True Tone but that might be something that is contributing the difference you are seeing.