phpBB v3.2.3 ships plupload v2.3.3.
It has been noted that when jpg images are uploaded from phones, the exif image rotation value is not honoured for images that are not large enough to require size reduction.
plupload v2.3.6 appears to have this anomoly fixed so that all uploaded jpgs are rotated according to the exif data.
There is some discussion at: https://www.phpbb.com/community/viewtopic.php?f=64&t=2429206 .
In my testing it appears that changing to plupload v2.3.6 is sufficient without requiring any change to phpBB core code.