I noticed that all my photos imported from my iPhone Camera Roll in Lightroom Mobile were marked with the metadata sign "+/-", although I hadn't made any modification in LR mobile (such like White balance, Highlights, Crop, or anything else...).
What does that mean?
It means there have been edit adjustments made to the image, possibly by a preset at import.
Thank you for your answer.
I didn't not LR Mobile was supposed to make edit adjustments to my images. I'm going to look after that.